Duplicate entries in Calendar

I have Mojave 10.14.6 & it works fine, I have latest iPad and iPhone OS - I enter a calendar entry on the computer and it replicates 2-3 times on my iPhone == I just went in to check it and changed the relevant calendar to another and one changed but the others did not == but if I delete one it deletes all. And yet, just now when I switched an entry back from the new calendar to the original one the two other entries were deleted. But I checked further and the next month has the same issue = and I can't figure out what the trigger is to have multiple items of the same thing.