A simple problem that i cannontcrack, and its just annoying me. The user must enter a lane number for an athlete

i want it to say:

On button Click
Store Lane in ListBox
Check If Lane already entered
MsgBox("Lane Already Entered")
else store lane
End If

how can i check if it has already been entered? what should the correct coding be

lstLane is the name of lstBox
iLane is Lane Number
do i need a counter?
Cheers