This is a standard advisory message which indicates what the system will do based on your choice.
The message starts off with "Tribute information submitted in this transaction..." This wording indicates the tribute information which the donor has supplied on the donation form.
"...will be overwritten with the information of the existing acknowlegee record." This wording indicates that the donor supplied information will be thrown out and that the information which is in the existing tribute record will be used instead.
If your goal is to link to an existing honor\memorial whereby you have information you expect to use for a common honoree, then you should click "Yes" on the advisory message, indicating that you wish to proceed using the information which is already on file and abandon the information which the donor has supplied.
Answering "No" indicates that you do not wish to use information you already have on file and instead, wish to create a new tribute. This will negate the link to tribute that you just completed.
For more information, please see Linking Tributes without creating duplicates.