-no added headers and none specified for removal. However, any data that was set
-by the &%address_data%& option in the current or previous routers is passed on.
-Setting the &%unseen%& option has a similar effect to the &%unseen%& command
-qualifier in filter files.
-
+no added headers and none specified for removal. For a &%redirect%& router, if
+a generated address is the same as the incoming address, this can lead to
+duplicate addresses with different header modifications. Exim does not do
+duplicate deliveries (except, in certain circumstances, to pipes -- see section
+&<<SECTdupaddr>>&), but it is undefined which of the duplicates is discarded,
+so this ambiguous situation should be avoided. The &%repeat_use%& option of the
+&%redirect%& router may be of help.
+
+Unlike the handling of header modifications, any data that was set by the
+&%address_data%& option in the current or previous routers &'is'& passed on to
+subsequent routers.
+.wen