Fix duplicate contributor names when generating changelog #993

Job	Run time
Coding Style \| 3.0	27s
Main Gem Specs \| RuboCop: master \| 2.7	3m 27s
Specs \| 2.6	35s
Main Gem Specs \| RuboCop: master \| 3.0	2m 57s
Specs \| 2.7	35s
Main Gem Specs \| RuboCop: v1.35.0 \| 3.2	1m 46s
Specs \| 3.0	19s
Specs \| 3.1	21s
Specs \| 3.2	18s
Specs \| head	11s
Specs \| mingw	1m 10s
Coverage \| 2.6	38s
Specs "modern" \| 3.0	15s
Specs \| jruby	41s
Specs \| truffleruby	1m 14s
Specs \| truffleruby-head	1m 33s
	16m 27s

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