Both possible fixes aren't very nice:
You can either (as usual) buy a commercial plugin, which adds to your JIRA costs and can cause problems with JIRA updates; or you can try a suggestion from 2014 where someone suggested changing JIRA code but didn't even test it (but then, you can just set up a Staging JIRA and try there.).
"JIRA MIsc Workflow Extensions" apparently contains a Workflow Validator "field has single value"
"ScriptRunner" apparently can do something similar, but is more expensive.
The code hack:
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