ich have found the CountIF formula. But i can't highlight cells if everything is the same except the last digit. For example here should be the cells with GA22515089-X highlighted
Use COUNTIF with the constart part of your strings and combined with wildcards. So count how many rows starts with same string. IF the count is greater than 1, it's a duplicate:
Formula:
=COUNTIF($A$1:$A$16,LEFT(A1,SEARCH("-",A1))&"*")>1