Fixed #1858 Adding same ExtGState multiple times

CHANGELOG.md

@@ -35,6 +35,7 @@ All notable changes to this project will be documented in this file.
 - Exporting DefineJPEG3/4 with alpha channel to PNG produced JPEG instead
 - AS3 package level const with function value - separate P-code for trait and method
 - Slot/const trait proper p-code indentation
+- [#1858] Adding same ExtGState multiple times
 
 ### Changed
 - Warning before switching deobfuscation is now optional

libsrc/gnujpdf/src/gnu/jpdf/PDFPage.java

@@ -454,7 +454,9 @@ public class PDFPage extends PDFObject implements Serializable {
     }
 
     public void addExtGStateResource(String resource) {
-        extGStateResources.add(resource);
+        if (!extGStateResources.contains(resource)) {
+            extGStateResources.add(resource);
+        }
     }
 
     /**