FULL PRODUCT VERSION :
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Darwin Rosss-MBP 15.6.0 Darwin Kernel Version 15.6.0: Mon Jan 9 23:07:29 PST 2017; root:xnu-3248.60.11.2.1~1/RELEASE_X86_64 x86_64

A DESCRIPTION OF THE PROBLEM :
Given the following:

interface A
interface B extends A
abstract class C implements A
public class D extends C implements B

Every method that appears in A is repeated twice in the JavaDoc for D. Note that adding the javadoc -private causes a similar issue in that the methods are listed as inherited from interface example.A and from class example.C.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create the following classes:

package example;
interface A {
   /**
   * JavaDoc for method in class A.
   */
   abstract void testA ( );
}

package example;
interface B extends A {
   /**
   * JavaDoc for method in class B.
   */
   abstract void testB ( );
}

package example;
abstract class C implements A {
   /*
   * Inherited JavaDoc
   */
   public final void testA ( ) {
      // Do nothing.
   }
}

package example;
public final class D extends C implements B {
   /*
   * Inherited JavaDoc
   */
   public final void testB ( ) {
      // Do nothing.
   }
}

Run the following command:
javadoc -d docs -sourcepath src -subpackages example

Note that the javadoc for the method "testA" appears twice in the JavaDoc for class D.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Since the signatures for the method "testA" are the same whether taken from class C or interface B, the javadoc for method testA should only appear once in class D.
ACTUAL -
The javadoc for the method "testA" appears twice in the JavaDoc for class D.

Note that adding the javadoc -private causes a similar issue in that the "testA" method is listed as inherited from interface example.A and from class example.C.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package example;
interface A {
   /**
   * JavaDoc for method in class A.
   */
   abstract void testA ( );
}

package example;
interface B extends A {
   /**
   * JavaDoc for method in class B.
   */
   abstract void testB ( );
}

package example;
abstract class C implements A {
   /*
   * Inherited JavaDoc
   */
   public final void testA ( ) {
      // Do nothing.
   }
}

package example;
public final class D extends C implements B {
   /*
   * Inherited JavaDoc
   */
   public final void testB ( ) {
      // Do nothing.
   }
}
---------- END SOURCE ----------