duplication of a variable when running two threads c#

I have a global variable "mc" which has a value of 100 after the operation. if you run the method in a stream, then everything is fine, but if you run 2 streams, the value of "mc" will become 200. how to fix it?

if (mc.Count > 1) 
{ 
System.Threading.Thread MyThread1 = 
new System.Threading.Thread(delegate () { select_page(0, mc.Count / 2, 0); }); 
MyThread1.Start(); 
} 
else 
{ 
potoc1 = true; 
} 
 
System.Threading.Thread MyThread2 = 
new System.Threading.Thread(delegate () { select_page(mc.Count / 2, mc.Count, 1); }); 
MyThread2.Start();

>Solution :

i had the same problem, add between running threads for

thread.sleep(50);