Psutil
is the best solution for this.
import psutil
processes = list(p.name() for p in psutil.process_iter())
# print(processes)
count = processes.count("<app_name>.exe")
if count == 1:
logging.info('Application started')
# print('Application started')
else:
logging.info('Process is already running!')
# print('Process is already running!')
sys.exit(0) # Stops duplicate instance from running