Consonant doubling is seen in a limited number of words ending in the consonants "b, t, d, k , l , s, z , m and  n "

The final consonant is doubled when a vowel-initial suffix is attached onto it. Consonant doubling is not seen when a consonant-initial suffix is attached. 

Examples ;

his [feeling ] ---> hissi  [ his/her feeling ]

sır [ secret ] ----> sırrı [ his/her secret ]

hat[ line ]  ----> hattı [ his/her/its line ]

Rab [ God ] ----> Rabbi [his/her God]

haz [ pleasure ] ----> hazzı  [his/her pleasure ]

tıp  [ [medicine ] ----> tıbbi  [medical]