diff --git a/index.js b/index.js
index 7bcc46a..d2bb878 100755
--- a/index.js
+++ b/index.js
@@ -12,7 +12,8 @@ var defaults = {
   fontViewportUnit: 'vw',  // vmin is more suitable.
   selectorBlackList: [],
   minPixelValue: 1,
-  mediaQuery: false
+  mediaQuery: false,
+  replace: true
 };
 
 module.exports = postcss.plugin('postcss-px-to-viewport', function (options) {
@@ -36,9 +37,16 @@ module.exports = postcss.plugin('postcss-px-to-viewport', function (options) {
 
       if (blacklistedSelector(opts.selectorBlackList, decl.parent.selector)) return;
 
-      var unit = getUnit(decl.prop, opts)
+      var unit = getUnit(decl.prop, opts);
+      var value = decl.value.replace(pxRegex, createPxReplace(opts.viewportWidth, opts.minPixelValue, opts.unitPrecision, unit));
 
-      decl.value = decl.value.replace(pxRegex, createPxReplace(opts.viewportWidth, opts.minPixelValue, opts.unitPrecision, unit));
+      if (declarationExists(decl.parent, decl.prop, value)) return;
+
+      if (opts.replace) {
+        decl.value = value;
+      } else {
+        decl.parent.insertAfter(i, decl.clone({ value: value }));
+      }
     });
 
     if (opts.mediaQuery) {
@@ -77,3 +85,9 @@ function blacklistedSelector(blacklist, selector) {
     return selector.match(regex);
   });
 }
+
+function declarationExists(decls, prop, value) {
+    return decls.some(function (decl) {
+        return (decl.prop === prop && decl.value === value);
+    });
+}
\ No newline at end of file
diff --git a/spec/px-to-viewport.spec.js b/spec/px-to-viewport.spec.js
index 23632e2..f530ac5 100644
--- a/spec/px-to-viewport.spec.js
+++ b/spec/px-to-viewport.spec.js
@@ -36,6 +36,13 @@ describe('px-to-viewport', function() {
 
     expect(processed).toBe(expected);
   });
+
+  it('should not add properties that already exist', function () {
+      var expected = '.rule { font-size: 16px; font-size: 5vw; }';
+      var processed = postcss(pxToViewport()).process(expected).css;
+
+      expect(processed).toBe(expected);
+  });
 });
 
 describe('value parsing', function() {
@@ -196,3 +203,15 @@ describe('minPixelValue', function () {
       expect(processed).toBe(expected);
   });
 });
+
+describe('replace', function () {
+  it('should leave fallback pixel unit with root em value', function () {
+      var options = {
+          replace: false
+      };
+      var processed = postcss(pxToViewport(options)).process(basicCSS).css;
+      var expected = '.rule { font-size: 15px; font-size: 4.6875vw }';
+
+      expect(processed).toBe(expected);
+  });
+});
\ No newline at end of file