If there be nothing new, but that which is
Hath been before, how are our brains beguil'd,
Which labouring for invention bear amiss
The second burthen of a former child.
Oh that record could with a backward look,
Even of five hundred courses of the sun,
Show me your image in some antique book,
According to the Oxford English Dictionary, “burthen” is an archaic form of “burden” or “load,” and can be used figuratively to mean “A load of labour, duty, responsibility, blame, sin, sorrow,” or literally “That which is borne in the womb.” 
 Shakespeare is playing with the idea that writing, or artistic creation, is a kind of reproduction. This  conceit  or extended  metaphor  was a popular one at the time, echoed by  Sidney  in the opening stanza of “Astrophel and Stella” where he compares his writers' block to being pregnant, “great with child to speak, and helpless in my throes.” 
 Continuing with the extended  metaphor  of Shakespeare “labouring for invention,” we can read this line as an anxiety that anything he (or we as a society) produce will only be the rebirth of something we already have. However, it can also be read as something truly new and grotesque—the return of a child which has already been born. So the concept of artistic innovation is ambiguous.
Sonnet 59