Parallel windings on a solenoid coil
"stronger" is not very scientific, it could mean many things. If you measure strength in Amp*Turns, your solenoid is twice as strong after the modification. If (as I understood from comments) you're considering the electromagnetic pull force which is proportional to (I * N)2, then your new solenoid is 4 times as strong.

Also, if your solenoid is small enough, its core may go into saturation before you reach maximum current. Increasing the current further in that case will have practically no effect on the pull force.