The double stroke roll is R R L L R R L L — two strokes per hand before switching. The trick: both notes within a hand should be equal, not loud-soft (a "diddle"). Use a controlled rebound rather than two separate full strokes.
Most beginners can play a clean RR LL slowly, but it falls apart when sped up — the second note of each pair becomes weaker. The exercises below use accents on the first of each pair to expose this weakness, then on every other pair to even it back out.
Rebound, Not Two Throws
Slow doubles are two deliberate wrist strokes; fast doubles are one throw plus a controlled bounce, with the fingers squeezing the second note up to full volume. Practice both mechanics separately: open strokes at ♩=60 where each note is its own decision, then bounce strokes at a pad-friendly tempo where you let the stick fall and catch the rebound. The transition zone between the two feels awkward for everyone, so pass through it patiently instead of avoiding it.
Watch for the press: burying the stick into the head chokes the rebound and the second note dies. If your RR sounds like "R-r", loosen the grip until the stick can breathe. On a pillow-soft surface doubles get harder and more honest; a minute a day there pays off fast.