diff --git a/package.json b/package.json
index fd80b969..cbcefe46 100644
--- a/package.json
+++ b/package.json
@@ -73,6 +73,7 @@
     "n3": "^1.16.2",
     "rdf-string": "^1.6.1",
     "relative-to-absolute-iri": "^1.0.6",
+    "sparqljs": "^3.7.3",
     "string-replace-loader": "^3.1.0",
     "webpack": "^5.69.0",
     "webpack-cli": "^4.9.2",
diff --git a/src/ldf-client-ui.js b/src/ldf-client-ui.js
index f9ac348e..1ed26d50 100644
--- a/src/ldf-client-ui.js
+++ b/src/ldf-client-ui.js
@@ -1,6 +1,8 @@
 /*! @license MIT ©2014–2016 Ruben Verborgh, Ghent University – imec */
 // jQuery widget for Triple Pattern Fragments query execution
 
+var SparqlParser = require('sparqljs').Parser;
+var SparqlGenerator = require('sparqljs').Generator;
 // This exports the webpacked jQuery.
 window.jQuery = require('../deps/jquery-2.1.0.js');
 var N3 = require('n3');
@@ -725,12 +727,14 @@ if (typeof global.process === 'undefined')
           return { type: type, value: datasource };
         }),
       };
-      var prefixesString = '';
+
+      let query = this.$queryTextsIndexed[this.options.queryFormat].val();
       if (this.options.queryFormat === 'sparql') {
-        for (var prefix in this.options.prefixes)
-          prefixesString += 'PREFIX ' + prefix + ': <' + this.options.prefixes[prefix] + '>\n';
+        // Add pre-defined prefixes to query and remove duplicates
+        const parsedQuery = new SparqlParser({ prefixes:this.options.prefixes, sparqlStar: true }).parse(query);
+        query = new SparqlGenerator({}).stringify(parsedQuery);
       }
-      var query = prefixesString + this.$queryTextsIndexed[this.options.queryFormat].val();
+
       this._queryWorker.postMessage({
         type: 'query',
         query: query,
diff --git a/yarn.lock b/yarn.lock
index c1df9ce2..9f1fcce7 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -8357,7 +8357,7 @@ sparqlalgebrajs@^4.3.7, sparqlalgebrajs@^4.3.8:
     rdf-terms "^1.10.0"
     sparqljs "^3.7.1"
 
-sparqljs@^3.0.0:
+sparqljs@^3.0.0, sparqljs@^3.7.3:
   version "3.7.3"
   resolved "https://registry.yarnpkg.com/sparqljs/-/sparqljs-3.7.3.tgz#075821d51ef4954284e36569503fe5558cfb71b0"
   integrity sha512-FQfHUhfwn5PD9WH6xPU7DhFfXMgqK/XoDrYDVxz/grhw66Il0OjRg3JBgwuEvwHnQt7oSTiKWEiCZCPNaUbqgg==