Adds a warning message (see #253).

Author: simonbrowndotje

structurizr-dsl/src/main/java/com/structurizr/dsl/StructurizrDslParser.java

@@ -4,6 +4,8 @@
 import com.structurizr.model.*;
 import com.structurizr.util.StringUtils;
 import com.structurizr.view.*;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 
 import java.io.File;
 import java.io.IOException;
@@ -20,6 +22,8 @@
  */
 public final class StructurizrDslParser extends StructurizrDslTokens {
 
+    private static final Log log = LogFactory.getLog(StructurizrDslParser.class);
+
     private static final String BOM = "\uFEFF";
 
     private static final Pattern EMPTY_LINE_PATTERN = Pattern.compile("^\\s*");
@@ -869,6 +873,9 @@ void parse(List<String> lines, File dslFile, boolean include) throws Structurizr
 
                     } else if (CONSTANT_TOKEN.equalsIgnoreCase(firstToken)) {
                         Constant constant = new ConstantParser().parse(getContext(), tokens);
+                        if (constants.containsKey(constant.getName())) {
+                            log.warn("A constant named " + constant.getName() + " already exists");
+                        }
                         constants.put(constant.getName(), constant);
 
                     } else if (IDENTIFIERS_TOKEN.equalsIgnoreCase(firstToken) && (inContext(WorkspaceDslContext.class) || inContext(ModelDslContext.class))) {