D-flat minor is a scale that exists in theory only. Such a key contains 8 flats, so yes, the B is flatted twice. Generally, keys only go up to 7 flats or sharps. The enharmonic equivalent C-sharp minor with 4 sharps would be preferable.
That is the signature for the key of Db. Or, it's relative minor, Bb minor.
The key signature of the C minor scale has three flats: B flat, E flat, and A flat.
That is C Minor
The key signature for F major has one flat (B flat) , while the key signature for F minor has four flats (B flat, E flat, A flat, and D flat).
Assuming the key signature has only one flat (B Flat) then the key is either F Major, or D minor.
One flat in the key signature indicates that the music is in the key of F major or D minor.
The key signature with only a B-flat is either F major or D minor.
The key signature with three flat signs is E-flat major. It contains the notes E-flat, F, G, A-flat, B-flat, C, and D. The relative minor of E-flat major is C minor, which also shares the same key signature.
D flat minor exists only in theory and not in practice, it's key signature would have six flats (Eb, Ab, Db, Gb Cb and Fb) and one double-flat (Bbb). It's notation can be expressed by the far simpler key of C-sharp minor - with only 4 sharps.
Only "B flat" is the key signature, then "C sharp" is the accidental.
Relative major and minor share the same key signature but a different tonic note (a tonic note is the first note of the scale or in solfege tonic is DO). For example, D flat major has five flats in it's key signature and b flat minor has 5 flats in it's key signature; therefore, D flat major and b flat minor are relative.
The key signature for D sharp minor would be the one for F sharp major: FCGDAE. You can also think of it as E flat minorwhich is related to g flat major in which the key signature would be BEADGC.