$ cat test
#!/bin/bash

# Create file and compress for testing..
touch test
tar -zcvf a_file.tar.gz test 

# List file
ls *.gz 

# Variable declaration
RESTORE_FF="a_file.tar.gz"

# -f : Return true value if file exists and regular file

# Double bracket, if file exist print message
if [[ -f $RESTORE_FF.* ]] ; then echo "Multiple files exists" ; fi

# Double bracket with not operator, if file not exist print message
if [[  !  -f  $RESTORE_FF.* ]] ; then echo "Multiple files exists" ; fi

# Single bracket, if file exist print message
if [ -f $RESTORE_FF.* ] ; then echo "Multiple files exists" ; fi

$ bash test 
test
a_file.tar.gz
Multiple files exists

$ sh test 
test
a_file.tar.gz
test: 16: test: [[: not found
test: 19: test: [[: not found

if ls  path/to/files*  &> /dev/null; then
    echo "files exist"
fi