I’ve come across some sheet music that has notes (Bass Clef) with 2 dots immediately below the note, as per the image below (right in the middle of the page):
What does this notation mean?
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The notes which have two dots below their heads also happen to have a half-time tremolo sign (the dash across the stem). This implies that you should play those notes as groups of two eight notes instead of one quarter, and the two dots are meant to make clear that both notes should be played staccato.
In the example below, the first measure is equivalent to the second:
SeuMenezes is right, and below is just the first example I found, even more extreme (bass clef and e flat major signature omitted):
Contrary to my assumption this is fine (as far as those abbreviations are ever), since Elaine Gould in Behind Bars writes in chapter Single-note tremolos, Repeated articulation:
Center on the notehead the number of articulation marks appropriate for the number of repetitions.
In this case the dots help to signal the scope for the slur, but I would have preferred it as text sempre portato.