Revert "deleted duplicate_entry_error.rs in favor of panic-handler/panic-handler-duplicate.rs"

This reverts commit ca3c094.

Author: spencer3035

tests/ui/duplicate_entry_error.rs

@@ -0,0 +1,18 @@
+//@ normalize-stderr: "loaded from .*libstd-.*.rlib" -> "loaded from SYSROOT/libstd-*.rlib"
+// note-pattern: first defined in crate `std`.
+
+// Test for issue #31788 and E0152
+
+#![feature(lang_items)]
+
+extern crate core;
+
+use core::panic::PanicInfo;
+
+#[lang = "panic_impl"]
+fn panic_impl(info: &PanicInfo) -> ! {
+//~^ ERROR: found duplicate lang item `panic_impl`
+    loop {}
+}
+
+fn main() {}

tests/ui/duplicate_entry_error.stderr

@@ -0,0 +1,16 @@
+error[E0152]: found duplicate lang item `panic_impl`
+  --> $DIR/duplicate_entry_error.rs:13:1
+   |
+LL | / fn panic_impl(info: &PanicInfo) -> ! {
+LL | |
+LL | |     loop {}
+LL | | }
+   | |_^
+   |
+   = note: the lang item is first defined in crate `std` (which `duplicate_entry_error` depends on)
+   = note: first definition in `std` loaded from SYSROOT/libstd-*.rlib
+   = note: second definition in the local crate (`duplicate_entry_error`)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0152`.