are rear surround sound speakers having same sound

Are rear surround sound speakers being feed the same signal? So there is no stereo signal at the back on 7.1 ht set up.
While there is no 7.1 source material, most decoders don't send identical signals to the surround back speakers.

THX-EX extracts surround back the same way a Dolby Surround decoder handles the center channel and applies a phase shift between the two speakers so that you get a wide image.

Lexicon Logic 7 is smarter about what goes there and can send completely different signals to the rear speakers.
Well no..not the "same" signal. I'd guess for a 7.1 system you have at least 8 signals.

Center Front - 1
Front - 2
Side - 2
Rear - 2
Sub - 1

If you're missing one pair, is the amp set for 5.1 output?
I think that there is only one "rear" (as opposed to "surround") signal. It can be sent to more than one speaker. It is a "derived" signal, probably a mix of the surround signals.
I believe if you have 7.1 source material they should be descrete channels and not mono. If you have 6.1 material it will be the same signal (mono) for both rear channels.

Some processors like Meridian will create their own side channel signal from 5.1 source material when you have a 7 channel system.

Check your processor manual to see what kind of capability it has.
My understanding of Logic 7, when used with a 5.1 source and driving a 7.1 speaker array for example, is it will process the two surround channels and make a determination of where to send the surround information. It reallocates the surround information across four surround speakers rather than two, depending on where it thinks the audio engineer wants the sound to emanate, to provide 7.1 discrete outputs. This allocation is dynamic, depending on signal content.

I do not know how DPLIIx works, but my guess is it's a similar concept to Logic 7 but with a different algorithm.