diff --git a/Library/Homebrew/dependency_collector.rb b/Library/Homebrew/dependency_collector.rb
index 4b8f9e8723..bd71d61b09 100644
--- a/Library/Homebrew/dependency_collector.rb
+++ b/Library/Homebrew/dependency_collector.rb
@@ -73,11 +73,11 @@ class DependencyCollector
   def ld64_dep_if_needed(*); end
 
   def zip_dep_if_needed(tags)
-    Dependency.new("zip", tags)
+    Dependency.new("zip", tags) unless which("zip")
   end
 
   def bzip2_dep_if_needed(tags)
-    Dependency.new("bzip2", tags)
+    Dependency.new("bzip2", tags) unless which("bzip2")
   end
 
   def self.tar_needs_xz_dependency?