NOTE: Test on in-portal with custom module installed.
1. Add these lines to modules/custom/install/project_upgrades.sql
```
# r2: Revision 2
# r3: Revision 3
```
2. Login to admin console as root
3. Go to {nav Tools > System Tools}
4. Press `Deploy` button
5. Confirm you see message contains:
```
Upgrading Database ...
# r1: Initial Project Task (#TASK_ID)
# r2: Revision 2
# r3: Revision 3
```
6. Add these lines to modules/custom/install/project_upgrades.sql
```
# r2: Revision 2 (duplicated)
```
7. Press `Deploy` button again
8. Confirm you see message
```
Verifying Database Revisions ... FAILED
Duplicate revision #2 found
```