In theory it is possible to run out musical note forms, but the infinite potential of key changes, tempos, instrumentation and interperetation mean that few people are sued for direct plaigarism. Famous cases like those of George Harrison, and more recently Robbie Wiliams are pretty rare, but similarities are very common. Part of the appeal of msuic to the senses of the listener is a degree of expectation, usually coverd by rhythm and chord changes, but occasionally by actual use of a known music pattern.