In general,  peak splitting in an indication of phase transformation to a lower symmetry. For example, from cubic to tetragonal, or from tetragonal to ortorhombic.

please upload your XRD plot, the information is also insufficient to explain.

Hi Gyass, you may have vibrations in your system, for example arising from some kind of  defect in the feedback loop in the driver circut of your stepper motor(s).  I asume that you have got an superposition of two XRD spectra; one of them  having a theta-scale and the other having a (theta + delta-theta)-scale. Please check you system.

I would also recommend to upload your data (image or the raw data are perhaps even better).  If you are afraid of spying do not say anything about the material. But a few word about the instrument and radiation used would perhaps help. Otherwise any advice is only speculation and does not help.

@ Gerhard: How often this happens?

Hi Gert, to my experience with stepper motors it often came up that vibrations enter into the experimental set up; especially in the 'half-step' mode. One has to properly set the  parameters of the motor driver. But I must admit that I hav'nt seen yet such mis-behavior in an XRD set up.

upload your xrd spectra then it will be quite clear..... and we will be able to answer;

or if u feel that it is because of system error then check for some other sample and check u still observe splitting or not

In general,  peak splitting in an indication of phase transformation to a lower symmetry. For example, from cubic to tetragonal, or from tetragonal to ortorhombic.