Or simply:

In[1]:= Split[listica][[All, 1]]
Out[1]= {a, b, a, b, a, b, a}

As an aside, although not particularly relevant here: note that Union always sorts its result into canonical order. If you don't need that behavior, DeleteDuplicates is often much faster:

In[2]:= RepeatedTiming[Union@RandomChoice[{a, b, c}, 10000000];]
Out[2]= {2.76162, Null}

In[3]:= RepeatedTiming[DeleteDuplicates@RandomChoice[{a, b, c}, 10000000];]
Out[3]= {0.674569, Null}

a faster version based on Split:

Map[First, Split[list]]

My pleasure Raspi, luck with your book!

Thank you so much @Henrik for the interesting coding alternative, which I wasn't able to figure out on my own (pattern matching). I learn, forget, need to relearn, forget again, omg.

Actually I am writing a book (a solutions manual to a high school math text) and I have included a link to this discussion. So your coding contributions add permanent value to my book, thanks again!!

Happy New Year @everyone!

Oh today I wasn't aware of the Split function, my restricted active vocabulary is embarrassing. @Mike i am still the original beginner, haha! Yes, this solves the problem beautifully, very succinct, thank you very much @Francisco, appreciated!

Great community here!