If you translate a document that has been previously translated, how similar would it be to the original document?

If you are talking about similarity then it should cover all the grammar & synonyms , but somethings can't be changed as the reader will definitely notice the similarity or else you'll roll out of subject

You don't expect to be similar. The important thing is that main points are there. There is always a gap in any original and non-original thing, it can be very narrow and hard to engage. Just gauge and scale. Finding the target is enough.

the similarity between the original text and the translated one depend on the narrow between the original culture and the target culture and the quantity of the information mentioned in the original text taking in our consideration that we should translate the important information exactly as mentioned in the text   

My translation will be similar by target meaning of the text, but surely different from anyone's other translation, as the word order, terminology and the emotional coloring of the text are individual

Certainly not a 100% ! The reason is something called "synonym". However, if you are to compare the meaning, then I'd say about 80-85% The quality might rise or drop depending on the person/machine that translated the original in the first place.