Question: What is the intention behind “ثنا فلان، ثنا فلان” (thana so-and-so, thana so-and-so) which you find in the chains of transmission of hadith?
Answer: The intention behind saying “thana so-and-so, thana so-and-so” is “haddathana so-and-so, haddathana so-and-so” (حدثنا فلان، حدثنا فلان). It is an abbreviation. Same thing with أنا which means “akhbaranaa.” Note that haddathanaa and akhbaranaa (حَدَّثَنَا وَأَخْبَرَنَا) mean “so and so narrated to us” and “so and so informed us.”
Suyuti wrote in Tadrib ar-Rawi:
The preponderance of abbreviations in writing the code sign of haddathana and akhbarana (حَدَّثَنَا وَأَخْبَرَنَا), for repeating it, circulating it, and appearing [often] to the extent it is not hidden nor ambiguous:
So they write from haddathana the ث and ن and ا, and omit the ح and د, IE ثنا. And perhaps the ث is omitted also and it is abbreviated on the pronoun, IE نا. And they write from akhbarana “أنا” which is the hamza and pronoun.