Asset manager duplicates asset types on project reopen after custom element is made

There is an issue where, in the Asset manager duplicates asset types on project reopen after custom elements are made for the "Primary Asset Types to Scan" array. THis issue does not appear to be a regression.

Versions tested: Issue occurs in all versions tested
4.16.3 - CL: 3561208
4.17.2 - CL: 3658906
4.18.0 - CL: 3709383

1. Create a new project (no template or starter content needed)
2. Open the project settings and select the asset manager on the left
3. Add a new array element to "Primary Asset Types to Scan" and check the "In Editor Only" box
4. Save and close the editor and reopen the project
5. Navigate to the asset manager again
   Result: There are now 5 Array elements in the "Primary Asset Types to Scan"
   Expected Result: There are still 3 array elements in the "Primary Asset Types to Scan"