J. S. Bach: Fantasia and Fugue in A minor

Interestingly, J. S. Bach composed two pieces that are both called Fantasia and Fugue in A minor.

The Fantasia and Fugue in A minor, BWV 904, was composed around 1725. Here's an excerpt from a review by Blair Johnston for Rovi Music:

Not to be confused with an earlier piece in the same key and going by the same title (BWV 944, ca. 1708), J.S. Bach's Fantasia and Fugue in A minor for harpsichord (or perhaps clavichord), BWV 904, is an early Leipzig-period work probably composed ca. 1725. The fantasia opens with a dramatic, thickly chordal phrase, built upon a descending bass line; ... The fugue itself is really a double fugue: the first subject -- a long, winding idea with nearly as many spaces as notes -- is put through the full expository works but is then cast aside in favor of a new subject -- this one pungently chromatic, steadily dripping downwards.

Not to be confused with his BWV 904, J.S. Bach's Fantasia and Fugue in A minor, BWV 944, is a piece for clavier composed between 1708 and 1717. Here's a description by James Reel from the AllMusic website:

The date of this piece can't be pinpointed beyond asserting that it comes from Bach's Weimar years, 1708 - 1717. The ten-bar fantasia is more complicated than it looks. On paper, it's just a series of chords taking less than a minute to play; in reality, the harpsichordist is expected to arpeggiate and improvise on the chords as lavishly as desired, exploring the chords' dissonances and harmonic surprises. The driven fugue, Bach's longest outside The Art of the Fugue, shares its basic theme with Bach's Fugue for organ in A minor (BWV 543). The restless, note-splattered music steadily percolates, thickening its texture with counterpoint derived from the main theme, modulating through several keys, and never dissipating its energy until the cadence of the final measures.

Sheetmusic at imslp.org