Added a duplicate alias check to the parser. Fixes #153.

e2e/scientists_queries.yaml

@@ -463,3 +463,17 @@ queries:
       - selected: ["?a", "?t"]
       - contains_row: ['<Carl_Sagan>', '1.8']
       - contains_row: ['<Noreena_Hertz>', '<Judaism>']
+  - query : duplicate-alias 
+    type: no-text
+    sparql: |
+      SELECT ?object (COUNT(?object) AS ?count) WHERE {
+              ?subject <Profession> ?object
+      }
+      GROUP BY ?object
+      ORDER BY DESC((COUNT(?object) AS ?count))
+    checks:
+      - num_rows: 836 
+      - num_cols: 2
+      - selected: ["?object", "?count"]
+      - contains_row: ['<Inventor>', '1616']
+      - contains_row: ['<Astrologer>', '43']

src/parser/ParsedQuery.cpp

@@ -280,7 +280,29 @@ std::string ParsedQuery::parseAlias(const std::string& alias) {
     }
 
     a._inVarName = alias.substr(start, pos - start - 1);
-    _aliases.push_back(a);
+    bool isUnique = true;
+    // check if another alias for the output variable already exists:
+    for (const Alias& other : _aliases) {
+      if (other._outVarName == a._outVarName) {
+        // Check if the aliases are equal, otherwise throw an exception
+        if (other._isAggregate != a._isAggregate ||
+            other._function != a._function) {
+          // TODO(florian): For a proper comparison the alias would need to have
+          // been parsed fully already. As the alias is still stored as a string
+          // at this point the comparison of two aliases is also string based.
+          throw ParseException(
+              "Two aliases try to bind values to the variable " +
+              a._outVarName);
+        } else {
+          isUnique = false;
+          break;
+        }
+      }
+    }
+    if (isUnique) {
+      // only add the alias if it doesn't already exist
+      _aliases.push_back(a);
+    }
   } else {
     throw ParseException("Unknown or malformed alias: (" + alias + ")");
   }