This is a textbook example of sperm competition. In many insects, sperm competition occurs within the spermatheca (female sperm storage organ) and it is often the last sperm deposited that achieves first fertilisation.
Various odonate species have evolved a more drastic method: to remove rival sperm from the spermatheca before depositing their own. This involves a complex morphological adaptation of the distal end of the male penis. Males use these structures to 'scoop' out the sperm from previous males.
To find out more about the evolution of female sperm storage organs and rivals' sperm displacement read the review by Aguilar et al. doi.org/10.1017/S0952836903004357
Image from Cordoba-Aguilar et al. 2003 Journal of Zoology