The Double post message means that you set the same status twice. It is not a problem. It just prevents 2 same statuses.
Suggestions 542 and 544 do not require any action on WikiTree. As it states on the help page:
Possible Causes
Missing link to Wikitree on Wikidata.
That means that based on relations on WikiTree and WikiData, WikiData page can ge linked to WikiTree profile. That will enable further checking of dates, locations and relatives.
In your case Blacker-90 seems to be the same person as Q75653311 Just the death date is wrong on Wikidata for 30 years. All wikidata info comes from the Peerage site where it is also wrong. And The peerage states as a source "Montgomery-Massingberd, Hugh. Burke's Irish Family Records. London, U.K.: Burkes Peerage Ltd, 1976." Correct way is to connect wikidata profile to wikitree. I did that now. It will cause next week a suggestion about death date. It can be marked as a false suggestion. It would also be good to add a Research note that WikiData and ThePeerage have wrong death date, so it doesn't get corrected in the future.
Simillar for Young-53280 it seems the name on WikiTree is wrong. According to husband's source her name is Barbara. and then it is the same as Q75653322, so I also linked it.
STEWART BLACKER, of Carrickblacker, High Sheriff co. Tyrone 1706, b. in 1671 ; m. 1704, Barbara, dau. of the Rev. Henry Young, A.M., and niece and heiress of William Latham, of Brookend, co. Tyrone