Difference between imasu vs iimasu?

I don't actually know Japanese and my searches online in dictionaries found nothing for either word. However, I do know (from reading lots of manga and watching lots of manga and listen to Japanese music) they are said differently. imasu is said i-mah-soo whereas iimasu is said i-i-mah-soo. iimasu has an extra syllable and sound in it from the second "i"