Index of non-unique element in data frame

Question

How can I extract the column names (or row and column index) of duplicate element in next data frame?

            V1         V2           V3           V4
PC1  0.5863431  0.5863431 3.952237e-01 3.952237e-01
PC2 -0.3952237 -0.3952237 5.863431e-01 5.863431e-01
PC3 -0.7071068  0.7071068 1.665335e-16 3.885781e-16

For example 0.5863431 is equal to 0.5863431, so "V1" and "V2" are the column names.

In that dataframe I want to get:

[1] "V1" "V2" "V3" "V4"

As you can see, looking rather only the result of the first row.

Second example:

            V1         V2          V3         V4
PC1 -0.5987139 -0.5987139 -0.03790446  0.5307039
PC2 -0.0189601 -0.0189601 -0.99315168 -0.1137136
PC3  0.3986891  0.3523926 -0.11045319  0.8394442

Result:

[1] "V1" "V2"

Answer 1

There may be a better way, but here's my take on it.

## coerce to matrix (if not already)
m <- as.matrix(df)
## find duplicates across both margins
d <- duplicated(m, MARGIN = 0) | duplicated(m, MARGIN = 0, fromLast = TRUE)
## grab the unique col names
colnames(m)[unique(col(d)[d])]

Examples: On your first data frame -

df1 <- read.table(text = "V1         V2           V3           V4
PC1  0.5863431  0.5863431 3.952237e-01 3.952237e-01
PC2 -0.3952237 -0.3952237 5.863431e-01 5.863431e-01
PC3 -0.7071068  0.7071068 1.665335e-16 3.885781e-16", header = TRUE)

m1 <- as.matrix(df1)
d1 <- duplicated(m1, MARGIN = 0) | duplicated(m1, MARGIN = 0, fromLast = TRUE)
colnames(m1)[unique(col(d1)[d1])]
# [1] "V1" "V2" "V3" "V4"

And on the second -

df2 <- read.table(text = "V1         V2          V3         V4
PC1 -0.5987139 -0.5987139 -0.03790446  0.5307039
PC2 -0.0189601 -0.0189601 -0.99315168 -0.1137136
PC3  0.3986891  0.3523926 -0.11045319  0.8394442", header = TRUE)

m2 <- as.matrix(df2)
d2 <- duplicated(m2, MARGIN = 0) | duplicated(m2, MARGIN = 0, fromLast = TRUE)
colnames(m2)[unique(col(d2)[d2])]
# [1] "V1" "V2"

Side note: Since your data contains all numeric values I would recommend beginning with a matrix instead of a data frame.