It is a technically correct structure , but it's a difficult and slightly ambiguous way of expressing something. The perfect tense is generally used to relate something that began (and may have occurred entirely) in the past to something that happened since -- and the usual relationship is that one event caused the other, or that one event preceded the other. Using the perfect structure twice in this manner seems to say either, "I received a diagnosis some time in the past and I did ...
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Picnicis this correct:Since I have been diagnosed I have done..."