Can Grep find duplicates in a list solving upper/lower case differences?

Question

The idea is isolate the info. Any grep that lead to these results will be enough to resolve the problem. Thanks.
actual list
option 1
option 2
option 3
halogen
halogen
halogen
Halogen
Halogen
Halogen
thebaica
therefore
therefore
their
Therefore
Therefore
them
they
they
therefore
They
They
Therefore
thermal
These
they
They
thin

Answer 1

I hope it'll be clear. Maybe I complicate the issue but I do it quickly.
1/ I apply the condition "All" to the entire text.
2/ I apply a char style "Capitales", so that I can convert all in Lower case (see ID menu).
3/ First Regex [for Option 1] (I forgot the screenshot):
in French:
Rechercher : ^(.+\r)\1+
Remplacer par :      (nothing)
Remplacer le format : select condition "Option 1"
4/ 2nd Regex [for Option 2]
4/ 3rd/4th/5th Regex [for Option 3 - in 3 steps]
5/ 6th Regex: I delete all the Text with condition "All".
6/ Now, we only have the Text with conditions Option 1/2 and 3 (see the colors).
7/ If you want, you can delete the 3 conditions:
DONE!
I use a lot with Grep the possibilities Conditions offer us!!! More complex but more fun!