Enhancing syntax checking with duplicate attribute detection

src/main/java/avatartranslator/AvatarBlock.java

src/main/java/avatartranslator/AvatarError.java

src/main/java/avatartranslator/AvatarSyntaxChecker.java

src/main/java/cli/Action.java

...vatartranslator/AvatarSpecificationSyntaxCheckerTest.java