The RS 120 headphones are "open aire" headphones which means that they allow sound in and out of the headphones. The headphones are designed like this to provide a more expansive sound scape and more nuance to the audio.
The "echo" that is sometimes experienced is caused because the same sound (ie the sound through the headphones and the other speakers) is being heard twice at slightly different times.
To resolve this issue there are a couple of options you can try:
1) Turn the volume in the headphones up until it covers the sound from the other speaker and eliminates the echo.
2) Turn the sound from the other speakers down until it gets low enough that the volume through the headphones is stronger than the sound from the other speakers and hence the headphones hide the echo.
NOTE: Sometimes a combination of option one and option two are needed to achieve the desired results.