public class Main(){
     public static void main(String[] args){
          System.out.print(" : :");
          System.out.print("a");
          System.out.print("b");
     }
}

Result looks strange at the NetBeans IDE's output window:
" : :ab : :ab
"		// that is not misprint - it's a new line

Changing print(" : :") to print(" : : :") results with " : : :ab : : :ab
"
that means result is duplicated somewhere

Something is happening when a string contains newline - result is correct.
My suggestion - string " : :" (hex value '20-3A-20-3A') means something specific or there are problems in the output buffer formatting or positioning.

Best Regards, udv.