This is caused by a column void forming at the head of the column or a blocked frit. This happens over time as the column begins to fail, but can happen more quickly if the flowrate and pressure are always very high. This causes the column bed to settle and a space to form. Contamination or incompatible mobile phases can also cause this.
Once upon a time there was a chromatographer new to the area of chiral separations. He injected multiple samples onto a preparative scale, $25,000, chiral column. These samples he dissolved in methylene chloride because he could not dissolve the in the methanol/hexane mix of the mobile phase. After the first separation and purification, the column performance began to fall. By the fifth sample, there was no stationary phase left. This particular phase was soluble in methylene chloride. The moral of the story, check the vendor specifications for the phase to ensure that the column is dying an untimely death.
If there is only on peak that is doubled, there may be a new partially coeluting compound in the mix. Diode array detection or MS is useful for this determination.
