The 1964 marriage took place in Lambeth whilst the 1866 marriage took place in Hastings. Makes me wonder if she had anything to hide; perhaps she had a husband who was still alive at the time of the first wedding who died before 1866 thus leaving her free to remarry legitimately. And going to Hastings to do it, with a slightly different name, would ensure that fewer awkward questions might be asked.
This is all supposition, of course, but could be one of many reasons for two weddings!
Regards
GS