disable diagnostic deduplication

src/tools/miri/src/lib.rs

@@ -143,4 +143,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zmir-keep-place-mention",
     "-Zmir-opt-level=0",
     "-Zmir-enable-passes=-CheckAlignment",
+    // Deduplicating diagnostics means we miss events when tracking what happens during an
+    // execution. Let's not do that.
+    "-Zdeduplicate-diagnostics=no",
 ];

src/tools/miri/tests/fail/const-ub-checks.stderr

@@ -10,6 +10,14 @@ note: erroneous constant encountered
 LL |     let _x = UNALIGNED_READ;
    |              ^^^^^^^^^^^^^^
 
+note: erroneous constant encountered
+  --> $DIR/const-ub-checks.rs:LL:CC
+   |
+LL |     let _x = UNALIGNED_READ;
+   |              ^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0080`.

src/tools/miri/tests/fail/erroneous_const2.stderr

@@ -10,6 +10,14 @@ note: erroneous constant encountered
 LL |     println!("{}", FOO);
    |                    ^^^
 
+note: erroneous constant encountered
+  --> $DIR/erroneous_const2.rs:LL:CC
+   |
+LL |     println!("{}", FOO);
+   |                    ^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 note: erroneous constant encountered
   --> $DIR/erroneous_const2.rs:LL:CC
    |