Definition of 'Dual Apper'
A potential mortgage borrower who submits two mortgage applications (here, "apper" is slang for application) simultaneously with different lenders, who are typically mortgage brokers, without the two lenders' knowledge.
A borrower will do this to try to protect themselves from a lender who plays games with loan fees and costs after the application has been taken.
Investopedia explains 'Dual Apper'
While clearly some unscrupulous lenders try to play games with borrowers by raising interest rates, increasing fees, adding charges or delaying loan processing after the loan origination process has started, "Dual appers" raise the cost of borrowing for all borrowers, since lenders must charge enough to cover all their costs, including aborted loan applications.
A better approach for borrowers is to demand to know a lender's fees for services and shop around before the loan application process begins.