"The men have the coats."
Translation:De mannen hebben de jassen.
The double consonant is to preserve the pronunciation of the vowel.
- If you were to say manen you get a long-a sound, while with mannen you get a short-a sound.
The same applies to jasen (although it is not a word) and jassen.
If you want to hear more what the differences are between the different sounds have a look on this website: http://www.heardutchhere.net/DutchPronunciation.html
It actually seems to resemble English a bit. That is, when English actually feels like playing by the rules. ;-)