My calendar keeps inviting coworkers twice, and I cannot tell which account is guilty

I have one personal calendar, one work calendar, and a volunteer-group calendar. On my phone they appear in one calm, color-coded view. Underneath, they seem to be fighting.

The problem started when a coworker told me she had received two invitations to the same planning call. One came from my work address. The other came from my personal address five minutes later. I had created the event once from my phone and added her once.

When I opened the event, it appeared on two calendars. Deleting one copy made the other disappear from my phone for a moment, then return. My laptop showed only the work copy. The volunteer calendar was not involved, except that its color is almost identical to my personal one and has made every screenshot harder to read.

I could remove all three accounts and add them again, but that feels like throwing the furniture out because I lost a key. Before changing anything, I plan to capture the event details on the phone and laptop, note the organizer address in each invitation, check which calendar is the phone's default, and create a test event with only my own addresses. I also want to see whether an old forwarding or subscription rule is copying events between accounts.

The social cost is small but real. Duplicate invitations make me look disorganized, and coworkers cannot tell which copy will receive updates. I have stopped adding people from my phone until I understand it.

What evidence would tell you whether this is a display duplication, a true second invitation, or an automation copying events? I would prefer a reversible investigation that preserves existing meetings and does not send a fresh wave of updates while I test.