R Under development (unstable) (2023-05-27 r84465 ucrt) -- "Unsuffered Consequences"
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> options("rgdal_show_exportToProj4_warnings"="none")
> library(sp)
The legacy packages maptools, rgdal, and rgeos, underpinning this package
will retire shortly. Please refer to R-spatial evolution reports on
https://r-spatial.org/r/2023/05/15/evolution4.html for details.
This package is now running under evolution status 0 
> data(meuse)
> # pick 10 rows
> n = 10
> set.seed(1357) # fix seed for exact reproduction of test:
> ran10 = sample(nrow(meuse), size = n, replace = FALSE)
> meusedup = rbind(meuse, meuse[ran10, ])
> coordinates(meusedup) = c("x", "y")
> zd = zerodist(meusedup)
> sum(abs(sort(zd[,1]) - sort(ran10))) # 0!
[1] 0
> zerodist(meusedup, unique.ID = TRUE)
  [1]   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18
 [19]  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36
 [37]  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54
 [55]  55  56  57  58  59  60  61  62  63  64  65  66  67  68  69  70  71  72
 [73]  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90
 [91]  91  92  93  94  95  96  97  98  99 100 101 102 103 104 105 106 107 108
[109] 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126
[127] 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144
[145] 145 146 147 148 149 150 151 152 153 154 155 140   9  26   7  90  17 133
[163]  82  35 116
> # remove the duplicate rows:
> meusedup2 = meusedup[-zd[,2], ]
> print(summary(meusedup2))
Object of class SpatialPointsDataFrame
Coordinates:
     min    max
x 178605 181390
y 329714 333611
Is projected: NA 
proj4string : [NA]
Number of points: 155
Data attributes:
    cadmium           copper            lead            zinc       
 Min.   : 0.200   Min.   : 14.00   Min.   : 37.0   Min.   : 113.0  
 1st Qu.: 0.800   1st Qu.: 23.00   1st Qu.: 72.5   1st Qu.: 198.0  
 Median : 2.100   Median : 31.00   Median :123.0   Median : 326.0  
 Mean   : 3.246   Mean   : 40.32   Mean   :153.4   Mean   : 469.7  
 3rd Qu.: 3.850   3rd Qu.: 49.50   3rd Qu.:207.0   3rd Qu.: 674.5  
 Max.   :18.100   Max.   :128.00   Max.   :654.0   Max.   :1839.0  
                                                                   
      elev             dist               om         ffreq  soil   lime   
 Min.   : 5.180   Min.   :0.00000   Min.   : 1.000   1:84   1:97   0:111  
 1st Qu.: 7.546   1st Qu.:0.07569   1st Qu.: 5.300   2:48   2:46   1: 44  
 Median : 8.180   Median :0.21184   Median : 6.900   3:23   3:12          
 Mean   : 8.165   Mean   :0.24002   Mean   : 7.478                        
 3rd Qu.: 8.955   3rd Qu.:0.36407   3rd Qu.: 9.000                        
 Max.   :10.520   Max.   :0.88039   Max.   :17.000                        
                                    NA's   :2                             
    landuse       dist.m      
 W      :50   Min.   :  10.0  
 Ah     :39   1st Qu.:  80.0  
 Am     :22   Median : 270.0  
 Fw     :10   Mean   : 290.3  
 Ab     : 8   3rd Qu.: 450.0  
 (Other):25   Max.   :1000.0  
 NA's   : 1                   
> meusedup3 <- subset(meusedup, !(1:nrow(meusedup) %in% zd[,2]))
> print(summary(meusedup3))
Object of class SpatialPointsDataFrame
Coordinates:
     min    max
x 178605 181390
y 329714 333611
Is projected: NA 
proj4string : [NA]
Number of points: 155
Data attributes:
    cadmium           copper            lead            zinc       
 Min.   : 0.200   Min.   : 14.00   Min.   : 37.0   Min.   : 113.0  
 1st Qu.: 0.800   1st Qu.: 23.00   1st Qu.: 72.5   1st Qu.: 198.0  
 Median : 2.100   Median : 31.00   Median :123.0   Median : 326.0  
 Mean   : 3.246   Mean   : 40.32   Mean   :153.4   Mean   : 469.7  
 3rd Qu.: 3.850   3rd Qu.: 49.50   3rd Qu.:207.0   3rd Qu.: 674.5  
 Max.   :18.100   Max.   :128.00   Max.   :654.0   Max.   :1839.0  
                                                                   
      elev             dist               om         ffreq  soil   lime   
 Min.   : 5.180   Min.   :0.00000   Min.   : 1.000   1:84   1:97   0:111  
 1st Qu.: 7.546   1st Qu.:0.07569   1st Qu.: 5.300   2:48   2:46   1: 44  
 Median : 8.180   Median :0.21184   Median : 6.900   3:23   3:12          
 Mean   : 8.165   Mean   :0.24002   Mean   : 7.478                        
 3rd Qu.: 8.955   3rd Qu.:0.36407   3rd Qu.: 9.000                        
 Max.   :10.520   Max.   :0.88039   Max.   :17.000                        
                                    NA's   :2                             
    landuse       dist.m      
 W      :50   Min.   :  10.0  
 Ah     :39   1st Qu.:  80.0  
 Am     :22   Median : 270.0  
 Fw     :10   Mean   : 290.3  
 Ab     : 8   3rd Qu.: 450.0  
 (Other):25   Max.   :1000.0  
 NA's   : 1                   
> dim(meuse)
[1] 155  14
> dim(meusedup2)
[1] 155  12
> dim(meusedup3)
[1] 155  12
> dim(remove.duplicates(meusedup))
[1] 155  12
> 
> proc.time()
   user  system elapsed 
   0.31    0.03    0.34