Merging .ts files QT Linguist

hi, I am translating the GUI of QGIS desktop software.
The current developping version is on transifex platform from which it can be extracted the corresponding .ts file.

the older versions of QGIS are archived on github.

I'd like to insert the missing strings translated in the newer version in the older version. Also some strings have been corrected in the new version and I'd like to replace them in the older.

I used lconvert in this form:

lconvert -i 2.18.ts 2.99.ts -o total.ts

But QT Linguist warns that in total.ts there are duplicated strings.

Is it possible to insert in the older version (2.18) only the strings (new translactions or variations) that are the same from the newer (2.99)

thank you in advance

stefano