There is a lot of cases like this, it is just that they are different words. Usually the difference is a nuance / formality / or generation thing.
See: What is the difference between "友" and "友達" and "友人" ? "友" vs "友達" vs "友人" ? | HiNative
and: What is the difference between "友人" and "友達" and "友" ? "友人" vs "友達" vs "友" ? | HiNative
The differences in usage are listed.
As for 友だち this is exactly the same as 友達, writing it like that is just a style choice and sometimes used to emphasize part of a word.