FYI the Quebecois use "dejeuner" as breakfast, "diner" for lunch, and "souper" for dinner.
In some areas of the USA that is also the case...in the Central-South East...I believe.
"You have just done lunch" pouquoi n'pas?
the verb is "to have lunch" so you better write you (have) just had lunch
Should it not be "You just have had your dinner" or "You just had had your dinner", particularly, when Duo doesn't accept the verb "to dine" and always insists on writing "to have dinner"?