The rules state:
- At the end of the game, each mask allows the controlling player to copy the scientific symbol of a green card present in one of the two neighboring cities.
- If a player possesses multiple [mask] symbols, he or she can only copy a given card once, and must thus copy as many different cards as they have [mask] symbols.
- A player can copy two identical cards (if they have enough masks) if both neighboring cities have a copy (copy a Workshop card in the left city and a second Workshop card in the right city).
Example: At the end of the game, Alex has 2 cards with the [mask] symbol. The player to his left only has one green card, with the compass symbol. The player to his right has played no green cards. Alex’s first [mask] therefore counts as a [compass] symbol, but his second [mask] is useless as there are no other symbols to copy. If the player to the left of Alex had had 2 green cards with a [compass] symbol each, Alex’s 2 masks could’ve counted as [compass].
So the wording is pretty clear. For each mask symbol you control, you may copy the symbol of a single green card owned by either adjacent player. Hence,
- You may not copy the same building, even if you have more masks than buildings.
- You may not copy the symbol on a leader, since they are not a building / green card.