When a 10+ energy weopon has 5 stars then you get 2 of 

Many of the weapons have a limit of 1 until you unlock 6star level and then you can use 2 of the weapon. 

The Rank 6 (or 6 Star) upgrade unlocks a second copy for any weapon with an energy level of 10 or higher. That's why the A-Coin cost for ranking up are equivalent to weapons with lower energy levels, because until R6 you only have use of one copy.