Can a PDA guess more than once? L = {aⁿ bⁱ aⁿ | i,n > 0 }
Here is a solution with one stack. 


Start inserting a's into the stack.
Stop inserting a's when we encounter the first b. Halt any operations with respect to the stack as long as we get b's.
Start popping stack contents immediately when the b's stop and a's start.