diff --git a/site/en/docs/user-manual.md b/site/en/docs/user-manual.md
index 7a567bc2f31f41..75aa2d974f8d99 100644
--- a/site/en/docs/user-manual.md
+++ b/site/en/docs/user-manual.md
@@ -1326,6 +1326,8 @@ The platforms that are available as execution platforms to run actions.
 Platforms can be specified by exact target, or as a target pattern. These
 platforms will be considered before those declared in the WORKSPACE file by
 [register_execution_platforms()](/rules/lib/globals#register_execution_platforms).
+This option accepts a comma-separated list of platforms in order of priority.
+If the flag is passed multiple times, the most recent overrides.
 
 #### `--extra_toolchains={{ "<var>" }}labels{{ "</var>" }}` {:#extra-toolchains}
 
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/PlatformOptions.java b/src/main/java/com/google/devtools/build/lib/analysis/PlatformOptions.java
index a217fc85bf4bf3..6d1c021fa8e15c 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/PlatformOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/PlatformOptions.java
@@ -76,15 +76,15 @@ public static boolean platformIsDefault(Label platform) {
   @Option(
       name = "extra_execution_platforms",
       converter = CommaSeparatedOptionListConverter.class,
-      defaultValue = "null",
+      defaultValue = "",
       documentationCategory = OptionDocumentationCategory.TOOLCHAIN,
-      allowMultiple = true,
       effectTags = {OptionEffectTag.EXECUTION},
       help =
           "The platforms that are available as execution platforms to run actions. "
               + "Platforms can be specified by exact target, or as a target pattern. "
               + "These platforms will be considered before those declared in the WORKSPACE file by "
-              + "register_execution_platforms().")
+              + "register_execution_platforms(). This option may only be set once; later "
+              + "instances will override earlier flag settings.")
   public List<String> extraExecutionPlatforms;
 
   @Option(