I'm probably in the minority here: when I was a manager, I was always totally fine with my employees moonlighting. Even if they didn't tell me and I found out later (as I did in one case). The only thing I've really cared about is whether they did good work.
I believe the main reason employers have an issue with moonlighters is that they view it as lack of loyalty. There may be other reasons, such as concerns regarding whether the employee can perform at 100% at two or more jobs, but I really do think that loyalty is the primary concern by a large margin.
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