Transposons can cause a gene duplication during exon shifting.
Exon shifting is where a gene is trapped between two transposons, during a transposition event. This results in the movement of the gene as well as the transposons.
If this transposition occurs over a replication fork during DNA replication, it is possible to duplicate a gene.
LINE proteins are also able to insert mRNA transcripts (after reverse transcription to cDNA) into chromosomes. This results in the production of a pseudogene, due to the lack of regulatory regions required for the gene’s expression.
The fate of a duplicated gene is different to the original copy, due to there being different selection pressures. Accumulating mutations in the copy results in a pseudogene forming - it is not expressed as a gene. Neofunctionalisation occurs when the gene gains a new function. Subfunctionalisation involves the specialisation of one copy of the gene.
The formation of a pseudogene is beneficial, preventing changes to transcriptional activity.