When uploading compounds, the application will not recognize tautomers as duplicates even with the 'Tautomer' rule enabled in the 'Uniqueness Rules' of the library's settings. This can happen if the option 'Identity' is enabled, since this option supersedes all other options.
Solution:
1. Log in as an administrator
2. Navigate to 'Configuration' > 'Materials' > 'Compounds' > 'Uniqueness Rules'
3. Disable the 'Identity' option