When you see a verb of this form, it means that you have a "doubled verb" (in Arabic - "الفعل المضعف").

مَـرَّ, دَلَّ, حَـجَّ, سَرَّ, حَلَّ ..

A doubled verb (الفعل المضعف) is a verb where the final two letters are identical and written together. However, these letters can be separated in different grammatical forms.

أنا مَــدَدْتُنَـحْنُ مَــدَدْنا

أنتَ مَــدَدْتَأنتِ مَــدَدْتِ

أنْتُمْ مَــدَدْتُـمْأنْتُنَّ مَــدَدْتُـنَّ

هــــو مَــدَّهي مَــدَّتْ

هم مَــدَّواهُنَّ مَــدَدْنَ

Example

دَلَّ عَلَيْهِ السِّيَاقُ(The context made it clear) The context indicated it