The check to prevent more than one banner in the same category having the same alias fails on new banner but works on update banner

Steps to reproduce the issue - Part 1

Create a banner with the name "test1"
Create a second banner with the name "test1"

Expected result

First Banner saved. Second banner gives an error message about duplicate alias

Actual result

Both banners saved with the same alias

Steps to reproduce the issue - Part 2

Create a banner with the name "test2"
Create a second banner with the name "test2"
Create a banner category with the name "cat2"
Open each banner and save them in the "cat" category

Expected result

First Banner saved. Second banner gives an error message about duplicate alias

Actual result

First Banner saved. Second banner gives an error message about duplicate alias