diff --git a/packages/less/src/less/tree/ruleset.js b/packages/less/src/less/tree/ruleset.js
index a3324cf07..f7aadd84b 100644
--- a/packages/less/src/less/tree/ruleset.js
+++ b/packages/less/src/less/tree/ruleset.js
@@ -295,8 +295,7 @@ Ruleset.prototype = Object.assign(new Node(), {
                 if (r.type === 'Import' && r.root && r.root.variables) {
                     const vars = r.root.variables();
                     for (const name in vars) {
-                        // eslint-disable-next-line no-prototype-builtins
-                        if (vars.hasOwnProperty(name)) {
+                        if (Object.prototype.hasOwnProperty.call(vars, name)) {
                             hash[name] = r.root.variable(name);
                         }
                     }
diff --git a/packages/less/src/less/visitors/to-css-visitor.js b/packages/less/src/less/visitors/to-css-visitor.js
index 54ddd6398..b27cdb0ca 100644
--- a/packages/less/src/less/visitors/to-css-visitor.js
+++ b/packages/less/src/less/visitors/to-css-visitor.js
@@ -303,19 +303,16 @@ ToCSSVisitor.prototype = {
         // remove duplicates
         const ruleCache = {};
 
-        let ruleList;
-        let rule;
-        let i;
-
-        for (i = rules.length - 1; i >= 0 ; i--) {
-            rule = rules[i];
+        for (let i = rules.length - 1; i >= 0 ; i--) {
+            let rule = rules[i];
             if (rule instanceof tree.Declaration) {
-                if (!ruleCache[rule.name]) {
+                if (!Object.prototype.hasOwnProperty.call(ruleCache, rule.name)) {
                     ruleCache[rule.name] = rule;
                 } else {
-                    ruleList = ruleCache[rule.name];
-                    if (ruleList instanceof tree.Declaration) {
-                        ruleList = ruleCache[rule.name] = [ruleCache[rule.name].toCSS(this._context)];
+                    let ruleList = ruleCache[rule.name];
+                    if (!Array.isArray(ruleList)) {
+                        const prevRuleCSS = rule.toCSS(this._context);
+                        ruleList = ruleCache[rule.name] = [prevRuleCSS];
                     }
                     const ruleCSS = rule.toCSS(this._context);
                     if (ruleList.indexOf(ruleCSS) !== -1) {
diff --git a/packages/test-data/less/_main/rulesets.less b/packages/test-data/less/_main/rulesets.less
index e81192dbc..f724ec71d 100644
--- a/packages/test-data/less/_main/rulesets.less
+++ b/packages/test-data/less/_main/rulesets.less
@@ -27,4 +27,5 @@
     }
   }
   font-size: 2em;
+  hasOwnProperty: blue;
 }