I have lived in England, Germany (BFO), Hong Kong (BFO), Zimbabwe, Zambia and Australia. In those countries I have noted, guerilla spelled with a single 'r'.

I figure a good source to see if guerilla is "acceptable" spelling, would be the London Times newspaper:-

http://www.timesonline.co.uk/tol/sitesearch.do?x=36&y=9&query=guerilla
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I also looked up the word's etymology, at Word Origins:-

http://www.word-origins.com/definition/guerilla.html

guerilla

Word History

*
Date of Origin 19th c.
Etymologically, a guerilla is a ‘little war’. English acquired the word during the Peninsular War (1808–1814) from Spanish guerrilla, which is a diminutive form of guerra ‘war’ (a word ultimately of Germanic origin, related to English war). In Spanish it still means ‘skirmish’, and until well into the 19th century it was used in English for a ‘war characterized by irregular skirmishing’ (the famous Times war correspondent William Russell, for instance, reported on 18 March 1862 that ‘Arkansas is now the theatre of a large guerilla’). The first recorded use of the word in its present-day sense is by the Duke of Wellington, the British military commander in Spain, in a despatch of 1809: ‘I have recommended to the Junta to set the Guerillas to work towards Madrid’.

When you plug in the word "guerrila" at the Word Origins website, this is what you get:-

Your search didn't return any results. Here are some suggestions {etc}

Simply because a word was derived from another language does not mean that its spelling need necessarily be kept. Try the origin of the word "pyjama" in the Word Origins site. It tells us that the word was derived from Hindi pāejāma; the word "dolphin" yields Greek delphís.
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I'll stick with the guerilla I have always known, thanks.
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