I know of one instance where I stole a Tag.
Terraformable Metal-Rich worlds are very scarce, almost as scarce as GGGs if considerably less spectacular. At one point, we explorers here on the forum were debating as to whether or not such worlds actually existed, since none of us recalled ever seeing one. Someone had compiled a list of such worlds that had been reported to EDSM, and posted it on the forum - about a dozen or so at the time - so I immediately set course for the nearest one to my then-current position, to check it out for myself. Sure enough, it existed - it wasn't an error in either the game or EDSM (like the "terraformable ammonia worlds" cropping up in the Codex, which were all Codex errors).
Yes, the planet existed - and it wasn't Tagged in-game. I seem to recall the EDSM data was at least several months old at the time I visited the system, I'd have to check my records for the precise timing.
I didn't then immediately rush back to base to steal the Tag - I continued on with my survey work, then handed the data in along with all the rest of the data, a month or two later.
So, you be the judge: did I "steal the Tag"? I didn't actively try to steal it, but it's not like I went out of my way to avoid stealing it. I needed to scan the planet, to verify its terraformability status, so the only practical way to permanently avoid stealing it would have been to manually sell the rest of my exploration data system by system, meticulously avoiding selling the stolen system, then going and getting myself blown up so I'd lose the data - not something I really wanted to do, since I'm trying to keep my deaths to a minimum. I shouldn't have to spend hours manually selling a hundred pages of data, then commit suicide, just to avoid becoming a Tag thief. I don't even know if the original first discoverer ever went back and sold their data - it's possible they just quit the game, and even if they had sold the data, it's most likely that the original discoverer never even knew they'd found something rare and special. But I freely admit I don't "deserve" my Tag on that planet, since I didn't find it.