@Anonymous
Yes this is perfectly possible in PowerApps...however, there comes new ways to consider this.
1) Do you want the user to make the entry and then get an error?
This is very "code-ish" and...who likes to see an error? You will need to put in the formulas to lookup the chosen item in the other list, and then Notify the user if it exists.
If(
IsBlank(Lookup(myList,
Name=NameDropDown.Selected.Name && Entry.Value=ItemDropDown.Selected.Value).ID),
submitInformation, // Nothing found that already exists
Notify("Release the dogs...you already have one of those in the list") //found something, alert user
)
2) Do you want the user to make the entry but then not be able to submit (disabled submit)?
Submit Button - DisplayMode property:
If(
IsBlank(Lookup(myList,
Name=NameDropDown.Selected.Name && Entry.Value=ItemDropDown.Selected.Value).ID),
Edit, // Nothing found that already exists, enable button
Disabled //found something, disable button
)
3) Do you want the user to not even see any entries that can not be chosen?
Entry DropDown Items property:
Filter(ListOfEntryOptions,
!(Value In Filter(myList,
Name=NameDropDown.Selected.Name).Entry
)
)
This would give you a filtered list of the available options that are not in the list already.
The user could only choose items that are not already there.
Of course, none of the above is meant to be copy and paste into functioning formulas. Adjustments to your scenario are needed. But, they can serve as a good starting point.
I hope this is helpful for you.
