There’s happiness 喜 (xǐ), and then there is double happiness 囍(also xǐ) !
The ‘double happiness’ term is really used in writing and especially used in connotation with marriage, as shown on this candy gift that each of us got from a colleague that is getting married soon. The heart-shaped modification of the bottoms of the happiness characters is artistic interpretation, of course. And I cannot quite find out for sure how old the use of the heart ♥ symbol is. Its uses started in Europe.