fix: Check if slot is taken after proposer check in sequencer

yarn-project/sequencer-client/src/sequencer/sequencer.ts

@@ -284,15 +284,6 @@ export class Sequencer extends (EventEmitter as new () => TypedEventEmitter<Sequ
       isPendingChainValid: pick(syncedTo.pendingChainValidationStatus, 'valid', 'reason', 'invalidIndex'),
     };
 
-    // Check that the slot is not taken by a block already (should never happen, since only us can propose for this slot)
-    if (syncedTo.block && syncedTo.block.header.getSlot() >= slot) {
-      this.log.warn(
-        `Cannot propose block at next L2 slot ${slot} since that slot was taken by block ${syncedTo.blockNumber}`,
-        { ...syncLogData, block: syncedTo.block.header.toInspect() },
-      );
-      return;
-    }
-
     // Check that we are a proposer for the next slot
     this.setState(SequencerState.PROPOSER_CHECK, slot);
     const [canPropose, proposer] = await this.checkCanPropose(slot);
@@ -303,6 +294,15 @@ export class Sequencer extends (EventEmitter as new () => TypedEventEmitter<Sequ
       return;
     }
 
+    // Check that the slot is not taken by a block already (should never happen, since only us can propose for this slot)
+    if (syncedTo.block && syncedTo.block.header.getSlot() >= slot) {
+      this.log.warn(
+        `Cannot propose block at next L2 slot ${slot} since that slot was taken by block ${syncedTo.blockNumber}`,
+        { ...syncLogData, block: syncedTo.block.header.toInspect() },
+      );
+      return;
+    }
+
     // We now need to get ourselves a publisher.
     // The returned attestor will be the one we provided if we provided one.
     // Otherwise it will be a valid attestor for the returned publisher.