In the Medium section of the diary, Tip.it has the following:

 

 

 

Obtain an anti-dragonbreath shield from Duke Horacio.

 

On the second level in Lumbridge castle, talk to Duke Horacio and tell him you seek a shield that will protect you from dragon breath. If you already have an anti-dragonbreath shield, you will need to drop it to get a replacement (if you bring with you and drop right before you talk to Horacio you can pick it up after receiving a replacement, thus resulting in you having two anti-dragonbreath shields!).

 

 

 

The Acheivement Diary actually says Try to Obtain an anti-dragonbreath shield from Duke Horacio.

 

 

 

If you already have one, you don't need to drop it, he will just tell you to search your bank more thoroughly for it, and you'll still complete the diary entry.

Hmm, when I did the diary I did it with the shield in my inventory and he did not let me complete the task until I dropped it. I did not try it with the shield in the bank however. And you are correct that it says Try to.

 

 

 

I am going to change but if I could get a couple more verifications that would be great! :D

this is true - you can do it with the sheild in your bank - it will just not give you a second one.

Hmm, when I did the diary I did it with the shield in my inventory and he did not let me complete the task until I dropped it. I did not try it with the shield in the bank however. And you are correct that it says Try to.

 

 

 

I am going to change but if I could get a couple more verifications that would be great! :D

 

 

 

For me, Horacio wouldn't even talk about a dragon, nor dragon shield until I had taken it out of the bank and dropped it. I did this diary soon after it was released, though so it is possible that this has been changed.

It was the same for me as cowman.... I did it last Thursday..... just out of curiosity, have you completed Dragon Slayer.