diff --git a/src/mca/schizo/base/help-schizo-base.txt b/src/mca/schizo/base/help-schizo-base.txt
index 70e38c2ada..f635a95358 100644
--- a/src/mca/schizo/base/help-schizo-base.txt
+++ b/src/mca/schizo/base/help-schizo-base.txt
@@ -96,8 +96,11 @@ An incorrect value for the "--stream-buffering" option was given:
 Valid values are limited to 0, 1, or 2. Your application will continue, but
 please correct your command line in the future.
 #
-[duplicate-value]
-WARNING: an MCA parameter was listed more than once on the command line, or
+[missing-envar-param]
+Warning: Could not find the environment variable: %s
+#
+[duplicate-mca-value]
+Error: an MCA parameter was listed more than once on the command line, or
 multiple times in one or more files, but with conflicting values:
 
   Param:  %s
diff --git a/src/mca/schizo/ompi/schizo_ompi.c b/src/mca/schizo/ompi/schizo_ompi.c
index c29584d379..543f2c7a64 100644
--- a/src/mca/schizo/ompi/schizo_ompi.c
+++ b/src/mca/schizo/ompi/schizo_ompi.c
@@ -17,7 +17,7 @@
  * Copyright (c) 2013-2020 Intel, Inc.  All rights reserved.
  * Copyright (c) 2015      Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
- * Copyright (c) 2018-2020 IBM Corporation.  All rights reserved.
+ * Copyright (c) 2018-2021 IBM Corporation.  All rights reserved.
  * Copyright (c) 2021      Nanook Consulting.  All rights reserved.
  * $COPYRIGHT$
  *
@@ -452,7 +452,7 @@ static int check_cache_noadd(char ***c1, char ***c2,
                 if (0 != strcmp(cachevals[k], p2)) {
                     /* this is an error */
                     prte_show_help("help-schizo-base.txt",
-                                    "duplicate-value", true,
+                                    "duplicate-mca-value", true,
                                     p1, p2, cachevals[k]);
                     return PRTE_ERR_BAD_PARAM;
                 }
@@ -1162,40 +1162,6 @@ static int parse_env(prte_cmd_line_t *cmd_line,
     }
     prte_argv_free(envlist);
 
-    /* now look for -x options - not allowed to conflict with a -mca option */
-    if (0 < (j = prte_cmd_line_get_ninsts(cmd_line, "x"))) {
-        for (i = 0; i < j; ++i) {
-            /* the value is the envar */
-            pval = prte_cmd_line_get_param(cmd_line, "x", i, 0);
-            p1 = strip_quotes(pval->value.data.string);
-            /* if there is an '=' in it, then they are setting a value */
-            if (NULL != (p2 = strchr(p1, '='))) {
-                *p2 = '\0';
-                ++p2;
-            } else {
-                p2 = getenv(p1);
-                if (NULL == p2) {
-                    free(p1);
-                    continue;
-                }
-            }
-            /* not allowed to duplicate anything from an MCA param on the cmd line */
-            rc = check_cache_noadd(&cache, &cachevals, p1, p2);
-            if (PRTE_SUCCESS != rc) {
-                prte_argv_free(cache);
-                prte_argv_free(cachevals);
-                free(p1);
-                prte_argv_free(xparams);
-                prte_argv_free(xvals);
-                return rc;
-            }
-            /* cache this for later inclusion */
-            prte_argv_append_nosize(&xparams, p1);
-            prte_argv_append_nosize(&xvals, p2);
-            free(p1);
-        }
-    }
-
     /* process the resulting cache into the dstenv */
     if (NULL != cache) {
         for (i=0; NULL != cache[i]; i++) {
@@ -1211,15 +1177,6 @@ static int parse_env(prte_cmd_line_t *cmd_line,
     prte_argv_free(cache);
     prte_argv_free(cachevals);
 
-    /* add the -x values */
-    if (NULL != xparams) {
-        for (i=0; NULL != xparams[i]; i++) {
-            prte_setenv(xparams[i], xvals[i], true, dstenv);
-        }
-        prte_argv_free(xparams);
-        prte_argv_free(xvals);
-    }
-
     return PRTE_SUCCESS;
 }
 
diff --git a/src/mca/schizo/prte/schizo_prte.c b/src/mca/schizo/prte/schizo_prte.c
index 1c55b38691..65f6a42f98 100644
--- a/src/mca/schizo/prte/schizo_prte.c
+++ b/src/mca/schizo/prte/schizo_prte.c
@@ -17,7 +17,7 @@
  * Copyright (c) 2013-2020 Intel, Inc.  All rights reserved.
  * Copyright (c) 2015      Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
- * Copyright (c) 2018      IBM Corporation.  All rights reserved.
+ * Copyright (c) 2018-2021 IBM Corporation.  All rights reserved.
  * Copyright (c) 2021      Nanook Consulting.  All rights reserved.
  * $COPYRIGHT$
  *
@@ -545,8 +545,12 @@ static int parse_env(prte_cmd_line_t *cmd_line,
                      char ***dstenv,
                      bool cmdline)
 {
-    int i, j;
-    char *p1;
+    int i, j, n;
+    char *p1, *p2;
+    char **env;
+    prte_value_t *pval;
+    char **xparams=NULL, **xvals=NULL;
+    char *param, *value;
 
     prte_output_verbose(1, prte_schizo_base_framework.framework_output,
                         "%s schizo:prte: parse_env",
@@ -567,6 +571,80 @@ static int parse_env(prte_cmd_line_t *cmd_line,
         }
     }
 
+    env = *dstenv;
+
+    /* now look for -x options - not allowed to conflict with a -mca option */
+    if (NULL != cmd_line && 0 < (j = prte_cmd_line_get_ninsts(cmd_line, "x"))) {
+        for (i = 0; i < j; ++i) {
+            /* the value is the envar */
+            pval = prte_cmd_line_get_param(cmd_line, "x", i, 0);
+            p1 = strip_quotes(pval->value.data.string);
+            /* if there is an '=' in it, then they are setting a value */
+            if (NULL != (p2 = strchr(p1, '='))) {
+                *p2 = '\0';
+                ++p2;
+            } else {
+                p2 = getenv(p1);
+                if (NULL == p2) {
+                    prte_show_help("help-schizo-base.txt",
+                                   "missing-envar-param", true,
+                                   p1);
+                    free(p1);
+                    continue;
+                }
+            }
+
+            /* check if it is already present in the environment */
+            for (n=0; NULL != env && NULL != env[n]; n++) {
+                param = strdup(env[n]);
+                value = strchr(param, '=');
+                *value = '\0';
+                value++;
+                /* check if parameter is already present */
+                if (0 == strcmp(param, p1)) {
+                    /* we do have it - check for same value */
+                    if (0 != strcmp(value, p2)) {
+                        /* this is an error - different values */
+                        prte_show_help("help-schizo-base.txt",
+                                       "duplicate-mca-value", true,
+                                       p1, p2, value);
+                        free(param);
+                        return PRTE_ERR_BAD_PARAM;
+                    }
+                }
+                free(param);
+            }
+
+            /* check if we already processed a conflicting -x version with MCA prefix */
+            if (NULL != xparams) {
+                for (i=0; NULL != xparams[i]; i++) {
+                    if (0 == strncmp("PRTE_MCA_", p1, strlen("PRTE_MCA_")) ||
+                        0 == strncmp("OMPI_MCA_", p1, strlen("OMPI_MCA_"))) {
+                        /* this is an error - different values */
+                        prte_show_help("help-schizo-base.txt",
+                                       "duplicate-mca-value", true,
+                                       p1, p2, xvals[i]);
+                        return PRTE_ERR_BAD_PARAM;
+                    }
+                }
+            }
+
+            /* cache this for later inclusion - do not modify dstenv in this loop */
+            prte_argv_append_nosize(&xparams, p1);
+            prte_argv_append_nosize(&xvals, p2);
+            free(p1);
+        }
+    }
+
+    /* add the -x values */
+    if (NULL != xparams) {
+        for (i=0; NULL != xparams[i]; i++) {
+            prte_setenv(xparams[i], xvals[i], true, dstenv);
+        }
+        prte_argv_free(xparams);
+        prte_argv_free(xvals);
+    }
+
     return PRTE_SUCCESS;
 }
 
diff --git a/src/tools/prun/prun.1.md b/src/tools/prun/prun.1.md
index 3fb14b81ca..ce4170621a 100644
--- a/src/tools/prun/prun.1.md
+++ b/src/tools/prun/prun.1.md
@@ -263,6 +263,11 @@ To manage files and runtime environment:
     before executing the program. Only one environment variable can be
     specified per `-x` option. Existing environment variables can be
     specified or new variable names specified with corresponding values.
+    If multiple `-x` options with the same variable name (regardless of value)
+    are provided then the last one listed on the command line will take
+    precedence, and the others will be ignored. The exception to this
+    is for PRTE_MCA_ prefixed environment variables which will report
+    an error in that scenario if any of the values differ.
     For example: `$ prun -x DISPLAY -x OFILE=/tmp/out ...`
 
 The parser for the `-x` option is not very sophisticated; it does not