ER-Model for Borrowing-Lending-Network?
Have a single User table.  Then if anything changes, such as a phone number, it changes for the lender and the borrower simultaneously without awkward reconciliation between the two tables.

Your Loan table will look something like this

Loan
----
LenderID        -- A foreign key pointing to User
BorrowerID      -- Also a foreign key pointing to User
ItemID


Then create a constraint on the table to ensure LenderID &lt;&gt; BorrowerID.  You don't say which RDBMS you're using but many of the major ones support this.  If anyone tries to lend an item to himself the DBMS will throw an error and refuse to store the data.