Databricks Unity Catalog feature failed error "feature tag key already exists on table"

TL;DR

Update the existing tag value in place instead of adding the tag again, or remove that tag key first and then set it, because Unity Catalog allows one value per tag key on a table and a second add for the same key is a conflict. Start with the Databricks Unity Catalog table tags panel and the feature table metadata, work on one affected call, record, or run first, and prove the fix there before running the full set. Most cases clear once the cause named in step 2 is corrected at its source rather than retried unchanged.

Databricks Unity Catalog feature failed error "feature tag key already exists on table"

Use this when

• You see this exact message or symptom: Databricks Unity Catalog feature failed error "feature tag key already exists on table"
• One run, record, call, or object is affected and you can name it
• A retry without changing anything reproduces the same result

Not for this skill when

• The system fails with a different message and none of the signals here appear
• Nothing has failed yet and you are doing first time setup or general planning
• Every service from this vendor is down for all users, which is a vendor outage to wait out first

Steps

1. Reproduce the failure once using the Databricks Unity Catalog table tags panel and the feature table metadata, and record the time, the workspace or account, and the one item it names. Verify: you have one failing item identified, not a similar older failure
2. Inspect the tag keys already on the table, the key in the failing call, and whether a notebook or job sets the same key earlier in the run. Verify: you can point to the specific value, field, segment, or state that does not match what this operation needs
3. Apply the fix at that source: Update the existing tag value in place instead of adding the tag again, or remove that tag key first and then set it, because Unity Catalog allows one value per tag key on a table and a second add for the same key is a conflict. Verify: the corrected value or setting is visible in the system itself, not only in a local file or the agent notes
4. Re-run the original operation on the single affected item first. Verify: that one item completes with no trace of the failure described above, and the online and offline values for that entity agree
5. Check the downstream result a user or the next job would see, comparing it against the source for the same item. Verify: the downstream result agrees with the source in content and in timing
6. Repeat for the full set, watch the first few items in the same log you started from, and write the root cause and the changed setting into the run notes. Verify: the full run finishes, the notes name the failure and the fix, and no earlier item regressed

Variant phrasings

Databricks Unity Catalog feature failed error "feature tag key already exists on table"

The subject-first phrasing of the same failure. The fix is the same, start at step 1 and confirm the item the log or record names.

Tool-first phrasing of this failure

The phrasing searchers use once they know which system raised it. It is the same underlying problem, not a separate bug.

Why does this keep happening after a retry

The repeat-occurrence phrasing. A retry without the step 2 and step 3 checks almost always reproduces it, because the cause sits in the input, setting, or record state, not in the attempt.

Compatibility: Databricks Unity Catalog governed tags, feature tables registered from Delta tables.

Why it happens

Tagging is usually spread across a creation notebook and a governance job, and both try to own the same key. The catalog protects the first value, so the second writer fails even though its value may be the newer one.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_Rt7sa-Q-QvYePuEnp4HIHg