What is this Chinese double happiness sticker.?

喜喜 It's not actually a character you can look up in the dictionary, but its 2 characters for happiness/joy put together. Traditionally they are used in marriage ceremonies and newly wed decorations for their home. It's like any other decoration in the house that signifies good wishes and "double happiness".

Double Happiness Sticker

Why don’t you look for it on the internet? I am sure you’ll find lots on Chinese double happiness stickers, Chinese wedding, wedding invitation, red packet, favor, and all related stuff. http://www.char4u.com/wedding

by double happiness, you mean this?
https://teachingchineseculture.wikispaces.com/file/view/%E5%8F%8C%E5%96%9C2.jpg

If yes, it's typically used as a wedding decor - hence, double happiness.

The word is made up of two "happiness" (or 喜), signifying both families are happy with this union in marriage.

If you want to read more about this character: http://chineseculture.about.com/library/weekly/aa120799a.htm

those are 对联 = dui-lian-42...

http://en.wikipedia.org/wiki/Duilian