Cannot able to sign twice by single user

I am using Adobe acrobat for signatures, 

Scenario, 

i need to get signature by 6 people, and in the 6 people, 2 people will do signture twice, like 

(i have 6 places to sign in the doc , line up like User1, User2, User3, User4, User2, User4) 

so user 2 and 4 have to do the signtures twice, 

when they do signtures for the first fiedls,  the signature get auto filled, but when it comes to second place in the document,  the system will not pick the signture automatically, user has to do manaul signatures, Why?

[Mod: Moved to the Adobe Acrobat forum]

Can you please help me if there is any chance to get sign auto picked.