Similar meaning:
* Previously: Implies something happened before the current moment.
* Before: Simple and straightforward.
* Prior to: More formal than "before."
* Earlier: Indicates a specific time in the past.
* Formerly: Indicates a state or condition that no longer exists.
* In the past: A general phrase for something that happened before.
More specific meaning:
* Already completed: For actions that are finished.
* Already decided: For decisions that are made.
* Already gone: For things that are no longer present.
* Already known: For information that is familiar.
* Already happening: For ongoing events.
Informal alternatives:
* Done: Short and casual.
* Over with: Used for finished tasks.
* Finished: Similar to "done," but more formal.
* Been there, done that: Used to indicate familiarity with something.
The best synonym will depend on the specific context and the nuance you want to express.