How to (grep) similar words ending or not in s?

The idea is to recognize identical words like:
car /cars
house/houses, etc.
Could not solve with look-behinds, etc.

We are working in an interactive grammar (spanish) where practically all the words, at least for our project, form plural with the addition of letter s. (Spanish do not have double ss ending words). Working with plurals is part of the goals.
The copy is a list (single words separated by hard returns already alfabetized):
Dardo
DardoS
Dedo
DedoS
Día
DíaS
Duda
DudaS
It is required that grep identify the singular or the plural when both words are the same.
(Some exceptions like the word Dios/Dioses= God will be resolved  by other means)
Thank you!

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