No, it's nothing to unusual. Convicts definately have a tendancy to be polygamous....though certainly not always. Have had many males pair up with more then one female at the same time; on a few occasions I have had males that were paired with 3 females at the very same time . Have had a few instances where males actually bred with 2 females at the same time and devided their time between both females.
Generally what is required are sex ratios with more females then males; enough space to keep the females apart and/or a male that is willing and able to keep the females apart from each other.
Never actually had a long term monogamous pair of convicts. Although males definately have a marked preference for certain females, when ever I have kept convicts in larger groups they always switched partners and usually quite frequently.