Денди-режим есть помимо нестопии еще на:
- нинтендуляторе
- ретрокопии
- пуНЕСе
Всюду такой эффект?

Думаю я, с такой темой нам всем прямиком на несдев дорога.

This sounds like a consequence of the design of the Dendy vs the NES. On the Dendy, they made the video hardware run at 50FPS instead of 60FPS. But they tried to keep the sound hardware running at the same speed.
The NES sound hardware has features such as volume envelopes, where the volume of the sound decreases over time at a constant rate. It also has frequency sweeps, where the frequency of the sound can change over time.
Most games call their sound playback code once per frame. So they update the sound registers at 50FPS. But the Dendy is still running the sound hardware features at 60FPS. This means that sweeps and volume envelopes are running at 6/5 of their normal speed.
For volume envelopes, the sound volume will decrease 1.2x as many times. It sounds like that game has the volume envelope looping feature turned on, so the drum sound restarts once it reaches zero volume. This happens before the game plays the next drum sound. Thus, you get the double-drum effect.
This isn't supposed to happen, and it's an error introduced by the Dendy. (But it's because the game isn't properly programmed, really nobody should be looping volume envelopes)