What does the icon, which is a red traingle with exclamation mark, mean in a text message?

A

AC Question

What does the icon which is a red traingle with exclamation mark mean in a text message

When I send a text message it then sends the message (or part of the message) again without me doing anything and the second duplicate message always starts with "nulle" or some form of "null". It is only doing it when I send a text message to one particular contact (who also has the same type of phone I have - HTC Desire626) but this contact is only see the one text message which is sent. Also I am receiving duplicate text messages from this same contact (but they are only sending it once). When I return to the list of contacts for text messages it shows the icon of a red triangle with exclamation mark on the right side next to the contact and preview of the last text sent and above where it shows the time the message was sent. What does the icon mean and why would it start doing this. It seems it only started within the last week
 

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