diff --git a/README.md b/README.md
index 955d819..c2cc8b2 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,7 @@ Default:
   selectorBlackList: [],
   minPixelValue: 1,
   mediaQuery: false,
+  replace: true,
   exclude: [] // ignore some files
 }
 ```
@@ -124,6 +125,7 @@ Default:
         - `[/^body$/]` will match `body` but not `.body`
 - `minPixelValue` (Number) Set the minimum pixel value to replace.
 - `mediaQuery` (Boolean) Allow px to be converted in media queries.
+- `replace` (Boolean) replaces rules containing rems instead of adding fallbacks.
 - `exclude` (Array or Regexp) Ignore some files like 'node_modules'
     - If value is regexp, will ignore the matches files.
     - If value is array, the elements of the array are regexp.
diff --git a/index.js b/index.js
index 88273bf..7e3be9f 100755
--- a/index.js
+++ b/index.js
@@ -14,7 +14,8 @@ var defaults = {
   selectorBlackList: [],
   propList: ['*'],
   minPixelValue: 1,
-  mediaQuery: false
+  mediaQuery: false,
+  replace: true
 };
 
 module.exports = postcss.plugin('postcss-px-to-viewport', function (options) {
@@ -54,9 +55,16 @@ module.exports = postcss.plugin('postcss-px-to-viewport', function (options) {
         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) {
@@ -104,3 +112,9 @@ 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 81ed2d6..3f3fa54 100644
--- a/spec/px-to-viewport.spec.js
+++ b/spec/px-to-viewport.spec.js
@@ -44,6 +44,13 @@ 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() {
@@ -293,6 +300,18 @@ describe('exclude', function () {
   });
 });
 
+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);
+  });
+});
+
 describe('filter-prop-list', function () {
   it('should find "exact" matches from propList', function () {
     var propList = ['font-size', 'margin', '!padding', '*border*', '*', '*y', '!*font*'];
@@ -342,3 +361,4 @@ describe('filter-prop-list', function () {
     expect(filterPropList.notEndWith(propList).join()).toBe(expected);
   });
 });
+