This article is the outcome of ground-breaking archival research into diaries kept by E. C. Bentley, the lifelong friend of G. K. Chesterton, between 1894 and 1900. I draw upon the diaries to construct a new account of the pair's youthful relationship, which challenges the orthodox biographical line on the circumstances of Chesterton's pained navigation of the fin de siecle. Combining analysis of the diaries with close reading of letters and manuscripts contained in the British Library's underexplored holdings of Chesterton's personal papers, I demonstrate that a continual struggle between identification and separation characterized the interaction of both men and that this unresolved duality operates as a vital context through which to understand Chesterton's subsequent literary and philosophical development. In particular, I build the case that Chesterton's aversion to philosophical scepticism, rationalist materialism, and biological determinism was first engendered by the traumas that punctuated the progress of his friendship with Bentley. I end by revealing the clandestine fictional presence of Bentley in two key texts of Chesterton's maturity - The Man Who Was Thursday (1908) and Manalive (1912) - again in contexts that rehearse an unresolved rebounding between affinity and antipathy, expressed aesthetically in a series of dialogic tussles between double figures. As this analysis demonstrates, the Bentley diaries represent far more than a mere scholarly curio. Rather, they offer an indispensable resource for the future biographer and/or critic wishing to develop a full and accurate account of Chesterton's life and work.