No you should never pour used buffer solutions back into the original bottle with fresh stock - always discard used buffer solutions after calibration as it is considered to be contaminated, and reusing it can affect the accuracy of your pH measurements.

Key points to remember:

Contamination risk - once used the buffer solution may be contaminated with substances from your sample, altering its pH value.

Proper procedure - when calibrating, always pour a small amount of buffer into a separate container and discard it after use.

Fresh buffer for each calibration - for best results, use fresh buffer solutions for each calibration.