Merging two level 10 weapons of the same tier

I wanted to know what will be the resultant evolved weapon if I use 2 weapons of level 10 to evolve into the next tier?
Example : I have 2 level 10 rare cannons. If I fuse both of them, will I get a level 20 Cannon or still i will get a level 11 cannon?

 

Level 11. It's no use levelling up your duplicates.

 

aaaahhh this is so unfair.

 

Not really, because it cost double the parts to go from 10-20 then 1-10