If no other keys are doing the same then the space button (bar) is probably sticking.
Check around the edges for any debris; bits of paper, crumbs, dried soft drink drops and so forth. Clean as necessary.
Try turning your keyboard upside down while the computer is off and taping the back side and ends with the palm of your hand. Just a few firm bumps. With a little shaking also.
Spread some paper out beforehand. You will be surprised at all the stuff that comes out.
Might help.
As a last resort you may be able to pop of the key cover and inspect things. However, by making keyboards un-cleanable the manufacturers can sell more of them once stuff gets really stuck somewhere. Be careful if you decide to take that approach.