The Rules for 2 Same Letters or 2 Similar Letters which Appear One After the Other
1. Mutamaathilain - this is when a letter appears with a sukoon, and after it comes the same letter again but with a fathah/kasrah/dhammah.
The rule is Idghaam, as in the first letter merges into the second.
2. Mutaqaaribain - when two letters with similar makhraj and qualities come one after the other, such as qaaf and kaaf. The rule here is also Idghaam.
3. Mutajaanisain - when two letters meet - and the first is saakin while the second is mutaharrik - and they have the same makhraj but different qualities, the rule is once again Idghaam: skip the first letter and go straight to the next.