Well, you can think of it like this. If you say I have had this experience before, it means that in the past, this happened to you, but if you say I had had this experience before now, it means that even in the past, you had already had the experience before some that in the past.
For example.
'I have had the experience before so I am more prepared today.'
Talking about the past with reference to the present
'I had had the experience before so I was more prepared on Friday.'
You had the experience at one point in the past so that at a later point which is still in the past, you had the experience to aid you. This is talking about the past with reference to a point closer in the past.
Example 2
Country is at war in 1984-85
Another war starts again today.
"we have had experience of war before so we are more capable this time"
Whereas
Country at war 1984-85
Country at war 1994-6
"When war started in 1994, we had had experience of war before, so we were more capable at that time.
Sorry I can't explain any better, but the intricacies of English are somewhat complex.
.^/¯/ Eaglehaslanded
\ /~
/// You, sir, are a wench - Scud
'' '' You, Sir, are a wrench - Reach