Are they truly set up as surrounds, I.e. showing up as +LS +RS in the room name with the soundbar, or have you possibly grouped two rooms together?

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater
 

https://support.sonos.com/en-us/article/group-and-ungroup-rooms?

The former should give you no echo, the latter would definitely give you an echo, as Sonos buffers the signal so it can be played in sync across all grouped rooms.