Updating Attribute in place results in duplicate attribute being appended to end of file

i'm fairly new to golang so please forgive my ignorance if the below code is incorrect. I'm trying to update an attribute in place within an tfvars file. However, whenever I set the attribute value, build the tokens and write to the file a duplicate attribute with updated values is appended to the bottom of the file. Below is some sample code:

newSlice := []string{"a", "b", "c", "d"}
file, err := os.OpenFile(`file.tfvars`, os.O_RDWR, 0755)
defer file.Close()
CheckError(err)
src, err := ioutil.ReadAll(file)
CheckError(err)
conf, diags := hclwrite.ParseConfig(src, `file.tfvars`, hcl.Pos{Line: 1, Column: 1})
if diags.HasErrors() {
log.Println(diags)
}
rootBody := conf.Body()
ctSlice, err := gocty.ToCtyValue(newSlice, cty.List(cty.String))
CheckError(err)
rootBody.SetAttributeValue(`attribute_to_update`, cty.ListVal(ctSlice.AsValueSlice())).BuildTokens(nil).WriteTo(file)

any help would be greatly appreciated, Thank you