are rear surround sound speakers having same sound

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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?
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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.

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.