C# :: Filtering Strings That Get Sent On A Event

I am writing a application that receives Event messages from a network device the problem I have is that the device sends some message 5 times per tiggered event on two of the events I like to use the rest of the messages are fine I need to create code to filter out the other 4 last messages that get displayed in a textbox the 5 message are identical I have added a timestamp to the sting and noted they are always 1 second apart so the format would be somthing like this:

11:23:01 - x - y
11:23:02 - x - y
11:23:03 - x - y
11:23:04 - x - y
11:23:05 - x - y

What I would like to do something like this, always append the first message to the textbox but ignore or even change the 4 messages that follow 1 second apart from each other that contain - x - y if any other message appears it should still append the text box as well

so the result i would like for the 5 event message as above should end like this:

11:23:01 - x - y
11:23:02 - custome message / ingnore append to textbox
11:23:03 - custome message / ingnore append to textbox
11:23:04 - custome message / ingnore append to textbox
11:23:05 - custome message / ingnore append to textbox