In v1.6 (don't know if this also affects former versions) there's a small bug in the blacksmith. If you accidentally do a double/triple click on the arrow range/attack upgrades, it will sometimes queue the same technology 2 or 3 times. It's nothing serious, since the upgrade effect will only apply once, but if you don't realize it you will lose time/resources. As far as I know this does only work for the arrow range/attack upgrades. I don't mind if it stays unfixed, just wanted to let you know (maybe it's an easy fix). The game crashes / desynch errors / being unable to join a restored game issues are way more important.

I noticed it the other day but wasn't able to reproduce it and thought it was a lag problem. While I was writting this message the reason why popped into my head and, indeed there's a bug in the triple tech code that may have been causing the issue. I can not test right now, though.

Indeed it's much easier to reproduce with a little lag. But even when playing vs AI I could reproduce it (not consistently though, you gotta click very fast).

Edit: I don't have the triple tech mod installed, so to my knowledge this shouldnt be the cause except if u did some changes in the main game data in order to enable the triple tech mod.

That's literally how it works. Same for Empire Wars.