Two Pips

In Great Expectations, Pip exists as both a child – the character of the story being told – and as an adult – the narrator of his own tale. This duality is especially apparent at the start of the novel, when Pip is at his youngest. Dickens uses this tool to impart upon the reader how much Pip matures and develops over his lifetime, to the point that he can look back on the events that seemed catastrophic or horrible and see benefit or humor in them.

EDIT: This contributes to the theme of the novel by showing substantial growth in an individual. The Victorian Era was a time of self-improvement and most people aimed to become better people - to grow as people - over the course of their lives. Having the two Pips shows in a very black and white fashion the way he changes by juxtaposing them throughout the novel.