iOS - Linker Error, Duplicate Symbol
Remove all of your classes from compile sources, then click the + button and search for the term '.m'. Highlight every class, then click add. Build and run again.
Similar error when importing third party library with multiple *.a files to IOS project. Removing ‘-all_load’ linker flag solved the problem.
Delete the file and choose "Remove Reference". Drag the file back again to the project and it should be ok. make sure there is no same files are there in multiple times.