There are some who like to collect watches in a particular sector, like all chronos, all time only, etc. I like more diversity in my collection so I try to buy pieces which are more different where no really isn't any overlap in my watches, ie: If I have a 44mm PVD Panerai w/ ETA movement, I will not get a 44mm PVD Panerai w/ in-house movement since it is too redundant for me. I won't even get a 47mm PVD Panerai either.
In the end, everyone has different tastes so their definition of a "collection" will always be different. But the best indicator of a good collection is whenever you have all your watches in front of you, if you don't have a big smile on your face, then u need to seriously reconsider your collection!
A collection should bring you joy instead of frustration.
Cheers,
Anthony