merge to たった1つの想い. Using proper tags is not "dumb". You can use whatever tags you want, last.fm will autocorrect it to it's proper title when it's merged. The whole point of last.fm is to suggest similar music based on what you listen to and what people with similiar tastes listens to. Smart people have this title correct as たった1つの想い and you're defeating the purpose of last.fm when you use the incorrect title. You separate yourself from people with similar tastes. Normally songs like this would be the least played spelling but because it's an anime song it's sadly the most used tag. Needs to be merged.
Also, http://www.kokia.com/disco/other/ lists the spelling as たった1つの想い and not たった１つの想い, which is why I used the regular styling for "1".