XTS objects with duplicate index have X in front of all index values.

General
2

Here's what I get when I run your code as a reprex:

library(xts)
#> Loading required package: zoo
#> 
#> Attaching package: 'zoo'
#> The following objects are masked from 'package:base':
#> 
#>     as.Date, as.Date.numeric
a <- xts(order.by = Sys.Date()-1:3,(8:10))
b <- xts(order.by = Sys.Date()-3:5,(10:12))
c = rbind(a,b)
c
#>            [,1]
#> 2019-04-12   12
#> 2019-04-13   11
#> 2019-04-14   10
#> 2019-04-14   10
#> 2019-04-15    9
#> 2019-04-16    8

Created on 2019-04-17 by the reprex package (v0.2.1)

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