When I went to a Spanish language class I was taught that the 4 letters capitalised in CaRoLiNa could be a very useful reminder that only the 4 letters can appear doubled up in Spanish words. Some examples are:
cc - dirección, lección
rr - perro, cerrar
ll - llave, pollo
nn connotar, innato
Carolina is very useful in spelling exercises, for example, words with double m in English are never spelt with double m in Spanish, similarly the rule applies to dd, ff, pp, ss etc. But recently I have come across several words in Spanish that do not conform to CaRoLiNa.
la sobreexplotación - over-exploitation
la contraataca - counter-attack
la brosse - brush (p.83 Dorling Kindersley Bilingual English Spanish Visual Dictionary)
el puzzle - jigsaw puzzle (p.273 Ibid)
Have you come across any other Spanish words with double letters apart from C R L and N ?