This happens a lot, especially with names that are translated. Russian names, for example, have been transliterated from Cyrillic in different ways over the years, like with Tchaikovsky...or Chaikovskii...or Tschaikofskii.

Which one should you use?

Check to see if your composer is listed in Grove Music Online, and if yes, use the version of their name that is the title of their article. For example, Pyotr Il′yich Tchaikovsky is how the Russian composer of six rocking symphonies appears in Grove.