Fix prefixing duplicate properties

Fixes parcel-bundler#537, closes parcel-bundler#553

Co-authored-by: LeoniePhiline <22329650+LeoniePhiline@users.noreply.github.com>

Author: devongovett

src/lib.rs

@@ -24019,6 +24019,24 @@ mod tests {
         ..Browsers::default()
       },
     );
+    prefix_test(
+      r#"
+      .foo {
+        -webkit-backdrop-filter: blur(8px);
+        backdrop-filter: blur(8px);
+      }
+      "#,
+      indoc! {r#"
+      .foo {
+        -webkit-backdrop-filter: blur(8px);
+        backdrop-filter: blur(8px);
+      }
+      "#},
+      Browsers {
+        safari: Some(16 << 16),
+        ..Browsers::default()
+      },
+    );
 
     prefix_test(
       ".foo { filter: var(--foo) }",

src/properties/prefix_handler.rs

@@ -95,11 +95,10 @@ macro_rules! define_fallbacks {
           $(
             Property::$name(val $(, mut $p)?) => {
               let mut val = val.clone();
+              $(
+                $p = context.targets.prefixes($p, Feature::$name);
+              )?
               if paste::paste! { self.[<$name:snake>] }.is_none() {
-                $(
-                  $p = context.targets.prefixes($p, Feature::$name);
-                )?
-
                 let fallbacks = val.get_fallbacks(context.targets);
                 #[allow(unused_variables)]
                 let has_fallbacks = !fallbacks.is_empty();