Board Game Arena

The rulebook doesn't specify it and I didn't find any clue to it, but is it really a permanent effect as implemented here on BGA? Based on what information?

The rules say: "Bonus tiles - Some provide an immediate effect that you carry out only once you gain the tile; others provide a permanent benefit throughout the game."

And for the bonus tile "1 duplicate counts", it says: "When delivering to El Paso, you may add exactly one duplicate to your total. Additional copies of this bonus tile allow you to add more than one duplicate."

Other bonus tiles have text like "Immediately gain 5 Dollars."

As the bonus tile "1 duplicate counts" does not say "Immediately", I believe it must be permanent.

Yeah, there are only immediate and permanent bonuses.
This bonus cannot be immediate and must therefore be permanent.

Also, when playing against good players, it's usually one of the least taken, only 1Vp...
(It does depend on the setup though)