It has the same meaning.
Does the expression "I'm telling you" mean the same as "I tell you" in the following definition by (the?) Free dictionary?
I tell you - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia.
Is there any difference in meaning? Is it widely used by native speakers?
If I were a native speaker of English, I would never shut up.)
It has the same meaning.
Both are used widely.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....