I'm telling you / I tell you

[englishhobby]

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?

 

[Tdol]

It has the same meaning.

 

[Grumpy]

Both are used widely.