sig extract and sig grep differ on duplicated md5sum

[bluegenes]

Some of our utilities may only keep *sig.gz files, leaving duplicates behind

sig extract does not keep duplicates:

sourmash sig extract -k 21 mm.dna.zip -o mm.dna-k21.zip

== This is sourmash version 4.8.4. ==
== Please cite Brown and Irber (2016), doi:10.21105/joss.00027. ==

loaded 428 total that matched ksize & molecule type
extracted 328 signatures from 1 file(s)

but this was originally created via sig grep, which does keep duplicate sigs

sourmash sig grep mammarenavirus spillover.dna.zip -o mm.dna.zip

== This is sourmash version 4.8.4. ==
== Please cite Brown and Irber (2016), doi:10.21105/joss.00027. ==

saving matching signatures to 'mm.dna.zip'
loaded 33566 total that matched ksize & molecule type
extracted 428 signatures from 1 file(s)

Confirmed this is not a reporting difference by listing files in *zip/signatures dir.

ref #2774 #1501 #2749