ERROR-Unable to copy file obj\debug\WFA1.exe

Error11Could not copy "obj\Debug\WFA1.exe" to "bin\Debug\WFA1.exe". Exceeded retry count of 10. Failed.

Error12Unable to copy file "obj\Debug\WFA1.exe" to "bin\Debug\WFA1.exe". The process cannot access the file 'bin\Debug\WFA1.exe' because it is being used by another process.

When I run the program firstly i run properly but when again running it then above errors shows with 10 warnings.
What should I do know?