Why do you need to tin gold plated terminals or leads twice? (electronics)?

Answer 1

The first solder that is heat melted on the gold surface has gold particles scavenged at the interface of the joint making a porous, non-hermetically sealed, microstructure. This structure is visibly duller, has poorer ductility, higher electrical resistance, and worst of all a much lower resonant frequency than a shiny properly fluxed solder joint. When vibrated, as on board a spacecraft or aircraft, dislocations and micro cracks have a higher probability to propagate and hence the joint might fall apart.

Wicking off that first gold scavenged solder then applying a second one makes a superior joint. The second application of solder does not directly come into contact with the bare gold surface because the previous tin has covered the gold up with an alloy that has a higher melting temperature than, say, the 180C eutectic melting temperature of SN62 (62% Sn, 36% Pb, 2% Ag).