Someone told me a D double flat was the same as a B#

Unfortunately, not a good guess.

A flat must be "attached to" a note. ("Associated with a note" is another way of saying it.)

A flat means "play the note one semitone lower."

A double flat means "play the not two semitones lower."

D♭♭ is E.

There are "character glyphs" commonly used to call for double sharps and double flats in music notation (and others less commonly used), but unfortunately those characters are not defined in the Unicode "Music Characters" section, so even if you have them it's not a good idea to try to post them here.

Using the note that's enharmonic (at the same pitch) with the double sharped or flatted note will be recognized by competent players, and will be played with the inflections in pitch and tonality appropriate to the scale and mode of the piece it appears in, so the symbols are actually quite rarely used except in notation of "artifacts" or by people named Igor or Darius (both of whom sometimes used "additional invented symbols" that require explanation even for the well educated theorists since they're unique to individual pieces and to the rather erratic moods of those composers).

John