I've only bought one TF twice by mistake (and that was a 3" titanium so you can see why I didn't care)
With most of my collection in storage, it's hard to do a quick shelf check. I'm pretty good at knowing what I have, but backup proof is good.
When I recently culled my collection I took a photo of what remained in each series and that's a helpful guide, and I plan to migrate that info to my spredsheet (eventually).
Last night I also had to audit my TF comics to remember which UK issues I'm missing before bidding on some eBay auctions - but now I have a definitive list of which issues I need to get and which I'd like to replace that it's in the cloud so I can get it if I stumble upon more for sale.
This thread might be worth a read
I had to look at old FB photos today on my phone in the shop to remember if I had bought the little Thrilling 30 G1 Starscream figure or not (turns out I had)
this has happend to me before when i opened a box to discover toys i had i didnt know or forgot i had, so digging up some old stuff from my Collection Thread