mapdist() returns too many lines with duplicated from/to

When passing a vector of to/from to mapdist that includes repeated pairs of to/from, the returned data frame re-duplicates repeated entries, and returns too many lines.

Here is code that demonstrates the problem.

``` r
library(ggmap)
library(dplyr)

# Create data frame with cities for map test, including repeated lines
cities <-c("Paris, France","London,England","Berlin, Germany")
df <- data_frame(from=rep(cities,2)) %>% mutate(to=lead(from,1)) %>% na.omit()

# Limit to the round trip of cities
roundTrip <- df[1:length(cities),]

# Repeat the first leg
tripRepeatedLeg <- df[1:(length(cities)+1),]

# The starting round trip trip gives the same number of results
roundTripDist <- suppressMessages(mapdist(roundTrip$from,roundTrip$to))
nrow(roundTrip)==nrow(roundTripDist)

# Trips with repeated legs return frames with additional rows
tripRepeatedLegDist <- suppressMessages(mapdist(tripRepeatedLeg$from,tripRepeatedLeg$to))
nrow(tripRepeatedLeg)==nrow(tripRepeatedLegDist)

```