It depends on what type of paint you are covering, how clean the surface is, and a number of other things.

More details would help, especially what type of paint is there (and age) alkyd (oil)?
Latex (water based)?

Gloss level?

Cleanliness level?

sorry.. you're right.
here are additional details:
-relatively clean
-matte/flat finish
-benjamin moore regal paint (new); water based 3 year (old)
-water based

-

Wash the wall and paint.

got it - thanks a lot for your advice!

For some reason washing surfaces before painting seems to have become unpopular and is not always done.

ANY paint adheres better to a clean surface.

If TSP is still available in your area (some places restrict it since it has a lot of phosphorous) it is still the best heavy duty cleaner for surfaces to be re-painted.

It does take a wash and a separate rinse, but makes the surface clean to accept a new coat of paint.

Caveat: if the old paint is dark (or a lovely little girl's pink), you'll need to prime or use more than one coat of the new. [We have a room with a pinkish under-tinge; we'll deal with it someday.]

It also depends on what color you are trying to cover. Primer will help with less coats of paint to cover.

One bedroom was vibrant yellow when I moved in. I painted over it with apple green, but I can still tell that yellow if fighting to get out.

In the basement was blue and white faux finish. I used 2 coats of primer and 2 coats of new paint and I can still tell that blue is there. Argh!

"It also depends on what color you are trying to cover. Primer will help with less coats of paint to cover. "

Using primer for this reason is not really saving you anything (time or money).

Quality primer cost almost exactly the same amount as paint, and it takes just as much labor to prime as to paint.