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The speaker is presumably setting these past events clearly into a previous time frame. There is no way of fully assessing this use without the surrounding context, however.
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Before we travelled back to Istanbul, Hazel and I had had a couple of holidays in Agastri and had made some Greek friends there.
hope this makes things clear why he used past perfect!
I agree but in text the past tense was not used. i think we should guess that it has happened before an another event in the past.
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On the contrary, the simple past appears as 'we travelled'. Because of the 'before' in the first clause, the past perfect is not necessary here, but the writer wishes to make clear that they had already travelled in the region previously. The past perfect here stresses that.
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The Past Perfect... 
Linear Mode
