using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
        string input = "blablablabla12345b\nMain Storage : C61\nC62 : 1215\nC61 : 1785\nblablablabla12345blablablabla";
 
        string pattern = @"(?: : (?<portion>[A-Z]{1,2}[0-9]{2})|^\s*(?<portion>[A-Z]{1,2}[0-9]{2}) :)(?!.*(?: : \k<portion>|^\s*\k<portion> :))";
        MatchCollection matches = Regex.Matches(input, pattern, RegexOptions.Singleline | RegexOptions.Multiline);
 
        foreach (Match match in matches)
        {
             GroupCollection groups = match.Groups;
             Console.WriteLine( groups["portion"].Value );
        }
	}
}

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