Polycythemia Vera (PV): The patient's elevated RBC, hemoglobin, and hematocrit levels, along with basophilia and a history of similar abnormalities, are highly suggestive of PV. The presence of these findings over time, despite a period of normalization, and the absence of secondary causes, further support this diagnosis. JAK2 V617F testing is warranted to confirm the diagnosis.

Essential Thrombocythemia (ET): Although the primary issue is elevated RBC and hemoglobin, ET can sometimes present with elevated hematocrit and basophilia. However, the significant elevation in RBC and hemoglobin points more towards PV.
Primary Myelofibrosis (PMF): PMF can present with elevated hematocrit, but it typically includes other features such as splenomegaly, bone marrow fibrosis, and more pronounced cytopenias or cytoses in other cell lines, which are not mentioned here.
Chronic Myeloid Leukemia (CML): CML could be considered due to basophilia, but the lack of other characteristic findings such as significant leukocytosis with a left shift and the absence of the BCR-ABL1 fusion gene mention make it less likely.

Secondary Polycythemia: Although the comprehensive metabolic panel did not reveal secondary causes, it's crucial to rule out conditions like chronic hypoxia (e.g., due to COPD, sleep apnea), renal cell carcinoma, or other tumors that could produce erythropoietin. These conditions could lead to similar laboratory findings and have significant implications for management.
Paroxysmal Nocturnal Hemoglobinuria (PNH): A rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, activation of the coagulation system, and impairment of bone marrow function. Although less likely given the presentation, PNH can sometimes mimic myeloproliferative neoplasms in its early stages.

Hereditary Polycythemia: Caused by mutations in the EPOR gene or other genes involved in the erythropoietin pathway, leading to increased sensitivity to erythropoietin and resultant polycythemia. This diagnosis would be considered if JAK2 V617F is negative and there's a family history of similar conditions.
Post-Polycythemic Myelofibrosis: A rare evolution of PV, characterized by the development of myelofibrosis following a period of polycythemia. This would be considered if there were signs of bone marrow fibrosis and a previous history consistent with PV.