Sorry cannot find any info on this unit.

I have a problem with my Hotpoint dishwasher, after it fills, washes and drains through the cycle, it keeps filling again while draining. After opening and closing the door, it seems to work fine though. On the last three fills/drains the same thing happens and a puddle of water appears on the floor underneath the dishwasher.
Not quite sure what the above means.
The first sentence seems to say that it runs the cycle OK and then starts filling/draining at the end.
But the last sentence seems to sat it does this during the cycle.
So I am confused.

I am guessing that this is a standard GE type washer but am not sure if you have a separate drain pump and I am assuming you do not.

The way most GE's work is that the unit fills, then does a wash cycle.
The drain solenoid (left hand side of the pump module) gets power for about 10 seconds and pulls the solenoid down. This moves the wash/drain flapper valve to the drain position and then the water pressure keeps the flapper in this position until the unit is empty (no water pressure). The flapper is then returned to the wash position by the springs on the solenoid arm assembly. The timer advances and the unit fills again for the next wash or rinse cycle.

Since opening the door resets the unit it sounds like the solenoid could be getting power when it should not (can be checked using a meter) or killing power stops the water pressure so the flapper can reset to the wash position (see below). If it is powered when it should not be it is a timer problem.

It could be the unit is not draining correctly so the flapper never reset to the wash position.
This could be caused by a drain plug.
Another possibility is the Piston & Nut. This is a valve at the back, bottom of the tub. It is behind the screened cover. Remove the cover (if there is one) and check the valve (it unscrews). If it is shot replace it. It is supposed to close during drain so that all the water goes out the drain and none of it is recirculated to the pump.

Hope the above is of some use.