Tašdid, تشدید literally, ‘gemination,’ is sometimes called “doubling.” In Persian, when in a single morpheme (word) a segment of adjacent identical consonants occurs (that is, a consonantal sound followed by a similar consonantal sound in a following syllable, creating a cluster; as in, [bb], [cc], [dd], etc.), and the segment is followed by a vowel (V) (that is, [bbV], [ccV], [ddV], etc.), a gemination occurs. In the Perso-Arabic writing system, the two sounds are contracted—that is to say, only one is written—and a [ ّ ] sign, called tašdid (šadda in Arabic, which literally means ‘to intensify’), is placed over the character as a (sort of) diacritic.

the prophet of Islam, name for men Mohammad محمّد
painter, image maker Naqqāš نقّاش
ink Morakkab مرکّب

Occasionally in both Arabic and Persian tašdid may occur terminally; as in, همّ hamm, Arabic for ‘care, solicitude,’ حیّ hayy, Arab for ‘alive,’ فرّ farr, Persian ‘fortune, glory, splendor.’ While this occurrence is normal in Arabic, it is very rare in Persian; for, as was explained, under normal circumstances, the segment must be followed by a vowel (which obviously is impossible in a final position); as in, بچّه bacce ‘child,’ فرّخ farrox ‘fortunate, blessed, happy.’ Therefore, cases such as farr have historical and etymological reasons for a terminal doubling. For instance, farr is a later development of xwarrah (~ xwarna-)—already with a medial geminated [-rr-], etc.

firstly, first of all avvalan اوّلاً
child, offspring bacce بچّه
sensitivity, susceptibility, being emotionally touched ta’assor تأثّر
haughtiness, presumption, arrogance takabbor تکبّر
pronunciation, articulation talaffoz تلفّظ
seriously jeddan جدّاً
valley, canyon, gape darre درّه
circular, round modavvar مدوّر
square; quadruple morabba’ مربّع
honored mošarraf مشرّف
vowel; sonant, sonic mosavvat مصوّت
determined, designated; fixed, assigned, scheduled,
stipulated; appointed, nominated
mo’ayyan معیّن

Words may have a combination of مدّ , تشدید , همزه , تنوین ; as in: اوّلاً , in which the و is mošaddad (‘having tašdid’; i.e., ‘geminated’), and the terminal alef has tanvin; تأثّر , in which there is a combination of hamze and tašdid, etc.

Also, bear in mind that the و in Persian numbers دوم dovom and سوم sevom must not be geminated. Therefore, the common pronunciation of these words as دوّم dovvom and سوّم sevvom is only on analogy with the geminated و in the Arabic اوّل awwal ‘first’ and اوّلاً awwalan ‘firstly’ (Persian avval, avvalan) (that is to say, dovom and sevom, without a geminated [v] are the correct pronunciations).