If two Haroof have one Mukhraj can we we make one of them sakin, like ''Waswaif Falyaboodoo".

Question
Asalamualikum....!!!!!
If two Haroof have one Mukhraj can we we make one of them sakin, like ''Waswaif Falyaboodoo".

Answer
Wa alaikum asalaam wa rahmatullahi wa barakatuh.

If both letters are the same exact letter, meaning they have the same articulation point (makhraj) and characteristics, and the first is saakin and the second vowelled, the first of the two like letters merges into the second.  This is called idghaam sagheer in Arabic, which means "small mergining" if translating.

If both letters are the same exact letter and both are vowelled, we do not merge them together in the reading of Hafs 'an 'Aasim. 

Please see:

http://www.abouttajweed.com/the-two-same-similar-close-and-far/the-two-the-same-the-two-similar-the-two-close-and-the-two-far-p-3.html

and

http://www.abouttajweed.com/the-two-same-similar-close-and-far/the-two-the-same-the-two-similar-the-two-close-and-the-two-far-p-2.html

The riwaayah of As-Soosiyy  'an Abi 'Amr does merge the first of two letters that are the same and both vowelled  into the second letter of the same kind if some conditions are met. This is called idghaam kabeer.   This though is a specialized study in the qira'aat. 
Wa assalaam alaikum.