diff --git a/arch/x86/adler32_ssse3.c b/arch/x86/adler32_ssse3.c
index 99ce795823..ae819d632e 100644
--- a/arch/x86/adler32_ssse3.c
+++ b/arch/x86/adler32_ssse3.c
@@ -54,7 +54,7 @@ Z_INTERNAL uint32_t adler32_ssse3(uint32_t adler, const uint8_t *buf, size_t len
         if (len < 16 + align_offset) {
             /* Let's eat the cost of this one unaligned load so that
              * we don't completely skip over the vectorization. Doing
-             * 16 bytes at a time unaligned is is better than 16 + <= 15
+             * 16 bytes at a time unaligned is better than 16 + <= 15
              * sums */
             vbuf = _mm_loadu_si128((__m128i*)buf);
             len -= 16;
diff --git a/inftrees.h b/inftrees.h
index eeae9c6ac4..ad2be151f2 100644
--- a/inftrees.h
+++ b/inftrees.h
@@ -44,7 +44,7 @@ typedef struct {
    examples/enough.c found in the zlib distributions.  The arguments to that
    program are the number of symbols, the initial root table size, and the
    maximum bit length of a code.  "enough 286 10 15" for literal/length codes
-   returns returns 1332, and "enough 30 9 15" for distance codes returns 592.
+   returns 1332, and "enough 30 9 15" for distance codes returns 592.
    The initial root table size (10 or 9) is found in the fifth argument of the
    inflate_table() calls in inflate.c and infback.c.  If the root table size is
    changed, then these maximum sizes would be need to be recalculated and
diff --git a/insert_string_tpl.h b/insert_string_tpl.h
index 4acd67fd62..c84617730a 100644
--- a/insert_string_tpl.h
+++ b/insert_string_tpl.h
@@ -43,7 +43,7 @@
 
 /* ===========================================================================
  * Update a hash value with the given input byte
- * IN  assertion: all calls to to UPDATE_HASH are made with consecutive
+ * IN  assertion: all calls to UPDATE_HASH are made with consecutive
  *    input characters, so that a running hash key can be computed from the
  *    previous key instead of complete recalculation each time.
  */
@@ -81,7 +81,7 @@ Z_INTERNAL Pos QUICK_INSERT_STRING(deflate_state *const s, uint32_t str) {
  * Insert string str in the dictionary and set match_head to the previous head
  * of the hash chain (the most recent string with same hash key). Return
  * the previous length of the hash chain.
- * IN  assertion: all calls to to INSERT_STRING are made with consecutive
+ * IN  assertion: all calls to INSERT_STRING are made with consecutive
  *    input characters and the first STD_MIN_MATCH bytes of str are valid
  *    (except for the last STD_MIN_MATCH-1 bytes of the input file).
  */
diff --git a/test/fuzz/fuzzer_minigzip.c b/test/fuzz/fuzzer_minigzip.c
index 819148d2b3..b59c69bd04 100644
--- a/test/fuzz/fuzzer_minigzip.c
+++ b/test/fuzz/fuzzer_minigzip.c
@@ -109,7 +109,7 @@ void gz_compress(FILE *in, gzFile out) {
 #ifdef USE_MMAP /* MMAP version, Miguel Albrecht <malbrech@eso.org> */
 
 /* Try compressing the input file at once using mmap. Return Z_OK if
- * if success, Z_ERRNO otherwise.
+ * success, Z_ERRNO otherwise.
  */
 int gz_compress_mmap(FILE *in, gzFile out) {
     int len;
diff --git a/test/minigzip.c b/test/minigzip.c
index 2615433ce7..537f64bbab 100644
--- a/test/minigzip.c
+++ b/test/minigzip.c
@@ -129,7 +129,7 @@ void gz_compress(FILE *in, gzFile out) {
 #ifdef USE_MMAP /* MMAP version, Miguel Albrecht <malbrech@eso.org> */
 
 /* Try compressing the input file at once using mmap. Return Z_OK if
- * if success, Z_ERRNO otherwise.
+ * success, Z_ERRNO otherwise.
  */
 int gz_compress_mmap(FILE *in, gzFile out) {
     int len;
diff --git a/zlib.h.in b/zlib.h.in
index 71f114a24f..45b6c8e092 100644
--- a/zlib.h.in
+++ b/zlib.h.in
@@ -924,7 +924,7 @@ Z_EXTERN int Z_EXPORT inflateSync(z_stream *strm);
      inflateSync returns Z_OK if a possible full flush point has been found,
    Z_BUF_ERROR if no more input was provided, Z_DATA_ERROR if no flush point
    has been found, or Z_STREAM_ERROR if the stream structure was inconsistent.
-   In the success case, the application may save the current current value of
+   In the success case, the application may save the current value of
    total_in which indicates where valid compressed data was found.  In the
    error case, the application may repeatedly call inflateSync, providing more
    input each time, until success or end of the input data.