Noted your query and inform you the following:
In contract, it arises when two or more persons in the same contract jointly promise to do the same thing, but also separately promise to do the same thing. For example, if A and B promise jointly and severally to pay £100 to C, then they are together under an obligation to pay £100 to C, but also individually they are under an obligation to pay the money to C. Performance by A or B discharges the obligation. In such a case, C is entitled to £100 in total and it can enforce the obligation in full against A or B or both. If C sues A and not B, it is open to A to claim a contribution from B.
Hope the example will clarify the matter.