How to nest ifcount into if?

=IF(J7 = "x"; (ifcount(E:E;E1)>1)); 1)

I would like to see if column J contains x or not and if yes check the numbers in column e, not to be able to put down the same number twice!

so i have 20 doors and if a door is used we put down an x i dont want to be able to use the same doors twice only if free!