One can write:
glielo porto, gliene riferisco, gliela scaldo, ...
lelo porto, mene incolse, tela scaldo, ...
Why gli is different in this respect? Where does this difference originate?
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Note that the equivalent of "te lo do" should be "*gli lo do" :-) At the end of the XIX century the form "glie lo do" is attested, for example in Cuore by Edmondo de Amicis - see http://forum.accademiadellacrusca.it/phpBB2/viewtopic.php@p=11486.html
The Accademia della Crusca says that the other forms were possibly not created to avoid confusion with the Italian words "melo", "tela", "mene"... but no formal production rule is really known.
The form glie exists only in combination with another pronoun, otherwise gli must be used. Writing glie lo is not frequent, but the form exists as testified by this NGram.
I believe that this usage only in combination contributed to the spelling in one word, whereas te lo dico is written divided. Note that these pronoun particles must be written attached to the preceding word when in enclitic position: dagliene, dimmelo.