This is a billon trachy of the Byzantine emperor Manuel I 1143-1180 and is Sear # 1966. While double strike errors are fairly common on Byzantine coins. I find this error very uncommon. The obverse depicts Christ enthroned and the letter IC-XC in the corners. As you know these are the Greek letters for Jesus Christ. The error on this coin is that instead of the IC-XC there is XC-XC. This is the first time that I have come across this mistake.

