Two issues descriptions below are based on these two comments: (1) the case with the Back button and (2) the case with unchecking the "Same as caption" checkbox

(1) the case with the Back button
Steps:

1. A user is on Describe step in Special:UploadWizard
2. A user doesn't see any warning about filling Caption/Description - CORRECT behavior
3. A user clicks on the Back button to return to the previous step
4. Then, a user click again the Next button to get to the Describe step
5. Both "Caption/Description is required" messages will be displayed.

Expected behavior: "Caption/Description is required" messages should not be displayed since a user hasn't yet attempted to publish a file.

(2) the case with unchecking the "Same as caption" checkbox - "Description is required" message
Steps:

1. A user is on Describe step in Special:UploadWizard
2. A user doesn't see any warning about filling Caption/Description - CORRECT behavior
3. A user unchecks the "Same as caption" checkbox ** - "Description is required" message appears

Expected behavior: "Description is required" message should not be displayed since a user hasn't yet attempted to publish a file.

(3) the case with unchecking the "Same as caption" checkbox and then checking it again - "Caption is required" message**
from @Sneha comment- https://phabricator.wikimedia.org/T366266#9850120

1. A user is on Describe step in Special:UploadWizard
2. A user doesn't see any warning about filling Caption/Description - CORRECT behavior
3. A user unchecks the "Same as caption" checkbox ** - "Description is required" message appears (the case from the previous scenario)
4. A user checks the "Same as caption" checkbox ** again - "Caption is required" message appears

Expected behavior: "Caption is required" message should not be displayed since a user hasn't yet attempted to publish a file.

(click on the animated gif below):