ZID CTLR MISSES INCORRECT READBACK RESULTING IN A CONFLICT BTWN A CLBING CLRJ AND AN MD80.

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I CANNOT POINT A FINGER AT ONE SPECIFIC THING THAT I BELIEVE CAUSED THE OPERROR. HOWEVER; THERE ARE SEVERAL FACTORS THAT I DO BELIEVE COULD HAVE BEEN 'CAUSAL' IN THIS AND A LOT OF THE ERRORS WE HAVE. MY RADAR ASSOCIATE WAS TOLD THAT ACFT #1 WAS STOPPED AT FL290. I REFLECTED THAT COORD IN THE DATA BLOCK AND WAS DISCUSSING ATC WITH THE RADAR ASSOCIATE (WHO IS A DEVELOPMENTAL) WHEN ACFT #1 CALLED ON (AWARENESS). WHEN ACFT #1 CALLED ON CLBING TO FL310 INSTEAD OF FL290; I ISSUED 3 SEPARATE PIECES OF TFC FOR HIM AT FL310 AND TOLD HIM I WOULD HAVE HIM AT THAT ALT (WHICH I THOUGHT WAS FL290) FOR AWHILE. MINS LATER; I REISSUED 1 OF THE 3 PIECES OF TFC AGAIN (THIS WAS ACFT #2) AT FL310. ACFT #1 'ROGERED.' I QUESTION 'WHY DID THE PLT NEVER QUESTION HIS ALT ASSIGNMENT.' MGMNT HAS TAKEN THE POS THAT HAVING 2 CTLRS (MINIMUM) AT THE SECTOR REGARDLESS OF TFC VOLUME IS IN THE BEST INTEREST OF SAFETY. HOWEVER; I HAVE FOUND NOT ONLY THROUGH THIS INCIDENT (BECAUSE THIS HAPPENS HUNDREDS OF TIMES A DAY WITHOUT INCIDENT); THAT WHEN THERE ARE 2 CTLRS AT A SECTOR AND THERE IS NO VOLUME TO SPEAK OF; CTLRS ARE HUMAN -- THEY LIKE TO TALK; EVEN IF IT HAS NOTHING TO DO WITH ATC. THIS DOES CAUSE YOU TO RELAX YOUR ATTN AND REFOCUS IT ON THE CONVERSATION REGARDLESS OF WHAT IT IS ABOUT. WHEN IT'S BUSY; WE'RE FOCUSED. WHEN IT'S NOT; WE'RE LESS FOCUSED (ESPECIALLY WITH SOMEONE THERE TO DISTRACT YOU).