I don’t know how to play this. How should I?
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There is really only one D there! It's written out with two 'voices', one with tails up, other with tails down.
On the face of it, it doesn't make a lot of sense, but that dotted minim needs to last for the whole bar - 3 beats, while the quavers keep playing on the other 5 places. It could have been written out with the first quaver D as a rest, and it would sound the same.
The idea is like satb, where there are 4 separate voices being put together, and when you count each part, it is clear there are 3 beats for each. It's a common, well accepted way of writing parts on piano.