Data Manager runs are not idempotent

[natefoo]

Describe the bug
If I run a data manager twice with the same inputs, the data manager is run twice, with the new data potentially (depending on the DM) overwriting the old, but with a new entry added to the location file.

Galaxy Version and/or server at which you observed the bug
Galaxy Version: all

To Reproduce
Steps to reproduce the behavior:

1. Install fetch_genome_all_fasta_dbkeys
2. Use the new DM to install a genome sequence.
3. Repeat the previous step.
4. The genome is overwritten in tool-data and two entries are added to all_fasta.loc.

Expected behavior
Either the new run overwrites the old one without adding a duplicate entry to the location file, or the DM refuses to run.

Screenshots

Additional context
This makes using the Ephemeris run-data-managers command idempotently or from a script more complicated.