When I tag master and push the the tag, two builds are triggered
Drone can only trigger a single build per webhook received from version control (eg GitHub). If multiple builds are triggered it would indicate multiple webhooks are being sent, in error, from version control.
the only step in the tagged event build, is clone
That when condition will never evaluate to true. You cannot specify a branch name for tag events because git tags are pointers to a commit sha, not a branch. A single commit can be pointed to by multiple branches or no branches; branches can be deleted or renamed or local only.
You will therefore need to remove the branch from the condition:
- branch: master