Weekday Formula where if the date is a Sunday it returns the original value

I am using the WEEKDAY function to return the week beginning and end dates, from a date in another cell, where beginning of the week is Sunday, and end of the week is Saturday.

It works where a date falls on any day of the week, apart from Sunday itself. If the date I need the week beginning and end date for is also a Sunday, and the beginning date itself, it's showing the date of the previous Sunday.

Example:

Examples A, C and D return the correct results. But when the date I am trying to get week beginning and end for is also on a Sunday, it's returning the previous week start and end dates. When I need it to return the same value essentially, as Sunday is the beginning of the week. 

Has anyone come across this before and know of a workaround? I'm completely stuck and would very much appreciate some help!