diff --git a/lib/capybara/selenium/driver.rb b/lib/capybara/selenium/driver.rb
index 993712567..f7d9e6fe3 100644
--- a/lib/capybara/selenium/driver.rb
+++ b/lib/capybara/selenium/driver.rb
@@ -192,7 +192,12 @@ def window_size(handle)
 
   def resize_window_to(handle, width, height)
     within_given_window(handle) do
-      browser.manage.window.resize_to(width, height)
+      # Don't set the size if already set - See https://github.com/mozilla/geckodriver/issues/643
+      if marionette? && (window_size(handle) == [width, height])
+        {}
+      else
+        browser.manage.window.resize_to(width, height)
+      end
     end
   end