I have just found on Ancestry the marriage record of Isaac Mayer and Barbara Strauss, born Simon, in Mainz in 1848.  Here's a loosely translated extract:
"On 19 January 1848 there appeared before me, etc ... Isaac Mayer, citizen of Mainz, hitherto living in Ingenheim, born in Kriegshaber in Bavaria on 31 January 1815, adult son of Joseph Mayer, teacher of religion, who died in Kriegshaber on 4 February 1818 and his wife Therese Bachmann who died in Kriegshaber on 15 April 1830.  The grandparents too are deceased: Joseph Mayer in Altenstadt on 24 January 1812, Rachele Landauer in Altenstadt on 2 June 1786, Elias Bachman [sic] in Altenstadt on 5 January 1803 and Fanny Bachman in the same locality [ebendaselbst] on 5 August 1840."
The record appears unusual in naming grandparents: I wonder if the explanation is that Isaac had no senior family members available to approve the marriage (the bride's mother, in contrast, is specifically recorded as living and present and as giving her approval). 
But I'm mainly posting about Isaac's father.  He is named Joseph Mayer - but so is the grandfather, who died only three years before Isaac's birth and must therefore have been living when Isaac's father Joseph was born.  I had thought there was a long-standing rule or custom among German Jews not to name children after living relatives, much less a living parent.  Can anyone suggest why that rule might have been breached in this case?  I'm aware that there has been recent discussion of this phenomenon on another thread, but that appears to centre largely around Eastern Jews and Sephardim, and I'm particularly focusing here on German Jewry.
Many thanks in advance,
Peter Lobbenberg in lockdown London, UK
peterlob@...