It centers on a wager of 100 sequins that Guglielmo's and Ferrando's fiancees, the sisters Dorabella and Fiordiligi, will prove unfaithful when put to the test.  
It is strikingly similar to Martin y Soler's opera L'abore di Diana, not surprising as the librettist was working on both operas at the same time.  It opens in a Naples caf?and soon moves to a villa, where the duet, "Ah guarda, sorella," is sung.  The doctor in Act I and the notary in Act II are none other than Despina, the maid, in disguise; though she is unaware of the nature of the charade.  That charade is engineered by the cynical Don Alfonso, who convinces two young men to assume the guise of Albanian nobleman to conduct a certain test.  It centers on a wager of 100 sequins that Guglielmo's and Ferrando's fiancees, the sisters Dorabella and Fiordiligi, will prove unfaithful when put to the test.  FTP, name this Mozart opera that translates as "So Do They All."
It is strikingly similar to Martin y Soler's opera L'abore di Diana, not surprising as the librettist was working on both operas at the same time.  
Cosi Fan Tutte (accept early So Do They All)